It’s kind of a fun phenomenon that no matter how thoroughly I research my history of science books, after they’re published I always find something that surprises me. In this case, I was tracking down some data on the physics of anomalous dispersion (more on that in another blog post), which led me to Robert Williams Wood’s textbook on Physical Optics, whose second edition came out in 1911. And in this book, I found a whole section on “invisibility of objects!”

This doesn’t change my overall narrative on the history of invisibility, as I’ve written about Wood’s 1902 paper on invisibility before, in which he experimentally tested a very curious (and certainly not true in general) hypothesis by Lord Rayleigh. I’ve also written about Wood’s science fiction novels before (two of them: here and here), and his fascinating series of physics lecture demonstrations. Wood was an all-around fascinating fellow, and he is one of a few physicists who got a mention in both my Falling Felines book and my Invisibility book!

Let’s take a look at Wood’s section on invisibility, because it is a fascinating glimpse into how physicists were even thinking about the possibility of invisibility over 100 years ago!

Wood’s Physical Optics book was his textbook on the physics of light, and like everyone who writes a textbook (including me), there is particular emphasis on topics he found most interesting. I’ve mentioned that he was interested in the phenomenon of anomalous dispersion, and great attention is paid to it in the book, along with his musings on invisibility. On the latter subject, he begins:

Opaque substances are seen by the light reflected from their surfaces; transparent substances in part by reflected light and in part by transmitted light. If we analyze carefully the appearance of a cut-glass decanter stopper we shall find it to be extremely complicated. Each facet reflects the image of some object in the room from its surface, and in addition to this shows some other object by refracted rays which have entered
some other facet, these latter being in general more or less spread out into a spectrum by dispersion. If the stopper is wholly or in part made of colored glass, the refracted rays passing through the colored portions are modified by absorption, and affect the appearance. This remarkable complex, we say, looks like a stopper, and unless we try to paint a picture of it, or have our attention drawn to the details, we are apt to regard its appearance as quite simple.

Here Wood notes two things. First, that a transparent object is seen through a much more complicated process than an opaque one. An opaque object is seen entirely through reflected light, while a transparent object is also seen via the light refracted through it. Second, he notes that we nevertheless have no problem discerning the shape of the object, even though there are all these complicated light processes involved.

We thus see that reflection, refraction, and absorption all play a part in making objects visible. It is interesting to examine into the conditions under which objects are invisible. If they are immersed in a medium of the same refractive index and dispersion, reflection and refraction disappear ; and if they possess in addition the quality of perfect transparency, they will be absolutely invisible, the light rays passing through them without any modification either in intensity or direction.

Noting that reflection, refraction and absorption all make objects visible, he turns to the most well-known method of making something invisible: refractive index matching. If we place an object into a medium with essentially the same refractive index, there will be no refraction at the surface of the object, and it will therefore not be seen.

Could a transparent solid be found whose refractive index was the same as that of air, objects made of it would be invisible.

Here I can’t help wondering if Wood was inspired by reading H.G. Wells’ The Invisible Man, which was published in 1897 and drew inspiration itself from the phenomenon of index matching:

“If a sheet of glass is smashed, Kemp and beaten into a powder, it becomes much more visible while it is in the air; it becomes at last an opaque white powder. This is because the powdering multiplies the surfaces of the glass at which refraction and reflection occur. In the sheet of glass there are only two surfaces; in the powder the light is reflected or refracted by each grain it passes through, and very little gets right through the powder. But if the white powdered glass is put into water, it forthwith vanishes. The powdered glass and water have much the same refractive index; that is, the light undergoes very little refraction or reflection in passing from one to the other.

“You make the glass invisible by putting it into a liquid of nearly the same refractive index; a transparent thing becomes invisible if it is put in any medium of almost the same refractive index. And if you will consider only a second, you will see also that the powder of glass might be made to vanish in air, if its refractive index could be made the same as that of air; for then there would be no refraction or reflection as the light passed from glass to air.”

The problem, however, and one that Wood was certainly aware of, is that the refractive index of air is very much the same as that of empty space, and we do not have any idea how to make a material that has the optical properties of nothing!

Wood gives a few examples of index matching that are now well-known to scientists:

The effect of immersing a transparent solid in a medium of similar optical properties is usually illustrated by dipping a glass rod into Canada balsam or oil of cedar, the immersed portion being practically invisible. A still better medium can be made by dissolving chloral hydrate in glycerine by the aid of heat. Only a little glycerine should be taken, as it is necessary to dissolve some eight or ten times its volume of the chloral before the solution acquires the right optical density. A glass rod, if free from bubbles or striae, becomes absolutely invisible when dipped in the liquid, and if withdrawn presents a curious appearance, the end appearing to melt and run freely in drops.

I linked above to my blog post on index matching, and if you’ve never seen it in action, it’s worth checking out the included video!

Wood then turns to his own personal study of invisibility, based on Lord Rayleigh’s hypothesis:

As a matter of fact, transparent objects are only visible by virtue of non-uniform illumination, as is pointed out by Lord Rayleigh in his article on optics in the Encydopoedia Britannica. If the illumination were the same on all sides they would be invisible, even if immersed in a medium of very different optical index. A condition approaching uniform illumination might, he says, be attained on the top of a monument in a dense fog. The author has devised a very simple method of showing this curious phenomenon, which, in brief, is to place the object within a hollow globe, the interior surface of which is painted with Balmain’s luminous paint, and view the interior through a small hole.

Lord Rayleigh’s hypothesis seems like it cannot possibly be true in general, but is apparently close to true in some cases, such as Wood’s experiment. Wood is basically just copying almost verbatim the words he wrote in his 1902 paper to describe his experiment, which is fair, since he wrote them! He continues:

The apparatus can be made in a few minutes in the following manner. A quantity of Canada balsam is boiled down, until a drop placed on cold glass solidifies. The Balmain paint, in the form of a dry powder, is stirred into the hot balsam until the whole has the consistency of thick paint. Two glass evaporating dishes of equal size are carefully cleaned and warmed, and coated on the outside with the hot mixture, which can be flowed over the glass, and by the dexterous manipulation of a small Bunsen flame made to cover the entire outer surface. Probably two perfectly plain hemispherical finger-bowls could be used instead of the evaporating dishes. As soon as the coating has become hard a small hole is cut through it through which the interior is to be viewed. If the lips of the dishes are placed together the interior can be seen through the small opening, but in this case the line of junction, which is always more or less dark, comes opposite the aperture, which is a disadvantageous arrangement.

If the inner surfaces be exposed to bright daylight, sun or electric light, and the apparatus taken into a dark room, a crystal ball or the cut-glass stopper of a decanter placed inside, will be found to be quite invisible when viewed through the small aperture. A uniform blue glow fills the interior of the ball, and only the most careful scrutiny reveals the presence of a solid object within it. One or two of the side facets of the stopper may appear if they happen to reflect or show by refraction any portion of the line of junction of the two hemispheres.

The apparatus would give better results if made on a larger scale, as the eye would not have to be brought so near the object. Two large wooden bowls would answer the purpose admirably. It is of the utmost importance to have a very thick and uniform coating of the paint, as otherwise the illumination is not uniform.

So Wood’s book doesn’t give much more detail than he had already provided in his paper, but the fact that he felt it worthwhile to devote a section to his book on invisibility makes it clear that he was still thinking about the subject. I think we can say that Wood was the first scientist to really, seriously think about the possibilities of making something invisible, at least in print! This is not surprising, as he was an incredibly imaginative fellow who also wrote science fiction.

In closing this post, let me share the full-color image that appeared in Wood’s book, which is absolutely gorgeous!

See those weird curvy lines in Figure 2a and 2b? Those are the regions of anomalous dispersion, which I will have to come back and blog about, because even the method of generating those figures is really neat!