September 1, 1886.] 



♦ KNOV/LKDGE ♦ 



331 



however, done some sen'ice in checking the rapid exhaustion 

 of our richest coal-fiekls. 



I should add to the above that some coal-seams are dry. 

 These are at present verj' rare, but will proljably become 

 more abundant hereafter, as we proceed deeper and deeper 

 in seai'ch of coal. As all the water encountered in sinking 

 coal-pit«. Wells, &c., is originally derived from the surface, 

 being rain that ha,s penetrated the earth's crust, there is 

 a limit beyond which it does not reach — the which limit is 

 determined by impermeability of strata due to compression 

 and by the continually increasing temperatme at increasing 

 depths. 



OPTICAL RECREATIONS. 



By a Fellow of the Royal Astronomical Society. 



i]N the series of papers which have appeared under 

 the above heading in these columns we have en- 

 deavoured to explain popularly the nature and 

 properties of light, the phenomena of reflec- 

 tion, refraction, interference, and dispersion, 

 the way in which we see, and the source and 

 origin of the innumeitible colours which we 

 behold around us. We have endeavoured to simplify our 

 exposition of optiavl phenomena generally by detailed and 

 illustrated descriptions of apparatus which may be made at 

 home by the veriest beginner ; and it now only remains for 

 the editor of this jouiTial to treat, as he has promised, of 

 the spectroscope and its applications, to render these essays 

 tolerably complete as an elementary introduction to the study 

 of optics. 



Our object, then, in this concluding chapter is to supple- 

 ment what we have previously said bj' an account of two 

 or three pieces of apparatus to which we omitted to refer in 

 the proper places, but which seem calculated to throw a 

 certain amount of light on those branches of the subject 

 which they are especially designed to illustrate. On p. 278 

 of vol. vi. of Knowledge will be found a description of the 

 action and construction of the stereoscope, and it is there 

 shown how the imager of a single object, when sufficiently 

 near to the eyes, are decided!}' different as seen by each of 

 them. It is further shown that it is by the convergence of 

 the eyes necessary to bring similar points (dissimilarly 

 situated) into coalescence that the sensation of solidity is 

 produced ; that, in fact, in looking at any very distant 

 object the optical axes of the eyeballs are parallel, and that 

 as an object approaches the observer those axes must become 

 more and more convergeiit. Now, the reader who has 

 thoroughly grasped our explanation will, on a little reflec- 

 tion, see that just as in algebra we can transfer any 

 quantity from one side of an equation to the other by merely 

 changing its sign, so it may be (and. as a matter of fact, it 

 is) possible so to present the two diverse images of a solid 

 body to the eyes as to affect the observer with the belief 

 that he is viewing a hollow one, and vice versd. This is 

 effected by the aid of a very simple instrument, much less 

 known than it deserves to be, and which is known as the 

 pseudoscope. It used to be exhibited at the now defunct 

 Polytechnic Institution. It is represented in plan (or hori- 

 zontal section) in the subjoined figure. 



Here are shown at a and b two prisms placed against a 

 block of wood, about two inches long and one and a half 

 inch wide, cut out in the centre at d to admit the nose. 

 The eyes are supposed to be looking at the globe c in the 

 dnection of the arrows. E, e, are brass-plates blackened, 

 which shut out the side light and assist in keeping the 

 prisms in position. A globe viewed through this simple 

 apparatus presents, for a reason which should be now per- 



fectly intelligible, the effect of a concave hemisphere ; a 

 hollow tea-cup, on the other hand, standing out as a globe. 

 The effect Ls very startling, but it sometimes requires a 

 certain amount of gazing to be produced. This pseudo-relief 

 may also be observed by regtirding the intaglio crest on a 

 signet ring illuminated by side light, through a lens of an 

 inch focus or so. Almost immediately the intaglio will 

 appaiently become a cameo, and the crest will stand up 



Fig. 1. 



above the surface of the stone with startling distinctness. 

 A cognate phenomenon often occurs in viewing lunar 

 craters in a telescope furnished with a solar diagonal eye- 

 piece, and ai-med with a tolerably high power, under which 

 circumstances the ci-aters put on the appe;irance of blisters 

 on the face of the Moon, presenting an absurd effect. 



In referring to the production of chromatic phenomena 

 in vol. ii. and elsewhere, we omitted to describe two very 

 simple experiments which exhibit them beautifidly and 

 effectively. For the first we are indebted to Sir John 

 Herschel, who illustrated the action of minute refracting 

 spheres by mounting the spores of the common puff-ball in a 

 film of oil between two plates of glass. When this arrange- 

 ment is held close to the eye, and a candle flame viewed 

 through it, beautiful concentric haloes appear. The second 

 little device was the invention of the late Rev. J.. B. Reade, 

 and was called by him the " iriscope." He took a circular 

 plate of glass, painted one side of it black, and, after care- 

 fully cleaning the other side, rubbed it over with a jjiece of 

 wet soap. This was, in turn, rubbed off with a clean soft 

 duster. A tube, about half an inch in diameter and a foot 

 long, was held about an inch from the centre of the soaped 

 plate, and the observer blew gently down this tube. The 

 result was that an immense number of minute particles of 

 moisture were deposited on the gla.ss, and these by inflection 

 decomposed the light and produced all the colours of the 

 spectrum. 



Proceeding now to our description of Maxwell's discs, and 

 the whirling table on p. 9.5 of the cuiTent volume, we may 

 add that what is done by this piece of apparatus may be as 

 easily effected by the so-called " Gorham's colour top "• — a 

 device of that well-known and most ingenious optical 

 physicist, Mr. Gorham, of Tonbridge, whose '• pupil 

 photometer" was described on p. 4G5 of our sixth volume. 

 This is, in fact, nothing more than a top made with a very 

 hea\T metal ring, like a gyroscope, on the face of which the 

 Maxwell's discs are laid. By touching the edge of one of 

 them as they rapidly rotate, a vai-ying amount of each 



