Water is invisible in water, alcohol in alcohol. But if alcohol be mixed with water the flocculent streaks of the alcohol in the water will be seen at once and vice versa. And in like manner the air, too, under favorable circumstances, may be seen. Over a roof heated by the burning sun, a tremulous wavering of objects is noticeable, as there is also over red-hot stoves, radiators, and registers. In all these cases tiny flocculent masses of hot and cold air, of slightly differing refrangibility, are mingled together.
In like manner the more highly refracting parts of non-homogeneous masses of glass, the so-called striæ or imperfections of the glass, are readily detectible among the less refracting parts which constitute the bulk of the same. Such glasses are unserviceable for optical purposes, and special attention has been devoted to the investigation of the methods for eliminating or avoiding these defects. The result has been the development of an extremely delicate method for detecting optical faults—the so-called method of Foucault and Toepler—which is suitable also for our present purpose.
Fig. 49.
Even Huygens when trying to detect the presence of striæ in polished glasses viewed them under oblique illumination, usually at a considerable distance, so as to give full scope to the aberrations, and had recourse for greater exactitude to a telescope. But the method was carried to its highest pitch of perfection in 1867 by Toepler who employed the following procedure: A small luminous source a (Fig. 49) illuminates a lens L which throws an image b of the luminous source. If the eye be so placed that the image falls on the pupil, the entire lens, if perfect, will appear equally illuminated, for the reason that all points of it send out rays to the eye. Coarse imperfections of form or of homogeneity are rendered visible only in case the aberrations are so large that the light from many spots passes by the pupil of the eye. But if the image b be partly intercepted by the edge of a small slide, then those spots in the lens as thus partly darkened will appear brighter whose light by its greater aberrations still reaches the eye in spite of the intercepting slide, while those spots will appear darker which in consequence of aberration in the other direction throw their light entirely upon the slide. This artifice of the intercepting slide which had previously been employed by Foucault for the investigation of the optical imperfections of mirrors enhances enormously the delicacy of the method, which is still further augmented by Toepler's employment of a telescope behind the slide. Toepler's method, accordingly, enjoys all the advantages of the Huygens and the Foucault procedure combined. It is so delicate that the minutest irregularities in the air surrounding the lens can be rendered distinctly visible, as I shall show by an example. I place a candle before the lens L (Fig. 50) and so arrange a second lens M that the flame of the candle is imaged upon the screen S. As soon as the intercepting slide is pushed into the focus, b, of the light issuing from a, you see the images of the changes of density and the images of the movements induced in the air by the flame quite distinctly upon the screen. The distinctness of the phenomenon as a whole depends upon the position of the intercepting slide b. The removal of b increases the illumination but decreases the distinctness. If the luminous source a be removed, we see the image of the candle flame only upon the screen S. If we remove the flame and allow a to continue shining, the screen S will appear uniformly illuminated.
Fig. 50.
After Toepler had sought long and in vain to render the irregularities produced in air by sound-waves visible by this principle, he was at last conducted to his goal by the favorable circumstances attending the production of electric sparks. The waves generated in the air by electric sparks and accompanying the explosive snapping of the same, are of sufficiently short period and sufficiently powerful to be rendered visible by these methods. Thus we see how by a careful regard for the merest and most shadowy indications of a phenomenon and by slight progressive and appropriate alterations of the circumstances and the methods, ultimately the most astounding results can be attained. Consider, for example, two such phenomena as the rubbing of amber and the electric lighting of modern streets. A person ignorant of the myriad minute links that join these two things together, will be absolutely nonplussed at their connexion, and will comprehend it no more than the ordinary observer who is unacquainted with embryology, anatomy, and paleontology will understand the connexion between a saurian and a bird. The high value and significance of the co-operation of inquirers through centuries, where each has but to take up the thread of work of his predecessors and spin it onwards, is rendered forcibly evident by such examples. And such knowledge destroys, too, in the clearest manner imaginable that impression of the marvellous which the spectator may receive from science, and at the same time is a most salutary admonishment to the worker in science against superciliousness. I have also to add the sobering remark that all our art would be in vain did not nature herself afford at least some slight guiding threads leading from a hidden phenomenon into the domain of the observable. And so it need not surprise us that once under particularly favorable circumstances an extremely powerful sound-wave which had been caused by the explosion of several hundred pounds of dynamite threw a directly visible shadow in the sunlight, as Boys has recently told us. If the sound-waves were absolutely without influence upon the light, this could not have occurred, and all our artifices would then, too, be in vain. And so, similarly, the phenomenon accompanying projectiles which I am about to show you was once in a very imperfect manner incidentally seen by a French artillerist, Journée, while that observer was simply following the line of flight of a projectile with a telescope, just as also the undulations produced by candle flames are in a weak degree directly visible and in the bright sunlight are imaged in shadowy waves upon a uniform white background.
Instantaneous illumination by the electric spark, the method of rendering visible small optical differences or striæ, which may hence be called the striate, or differential, method,[112] invented by Foucault and Toepler, and finally the recording of the image by a photographic plate,—these therefore are the chief means which are to lead us to our goal.