*588 



SCIENCE. 



Science, sion of the conversion of a cameo into an intaglio, 

 Curiosities and of an intaglio into a cameo, by an inverting eye- 

 *" piece, is the result of an operation of our own minds, 

 ^^y^*' whereby we judge of the forms of bodies by the know- 

 ledge we have acquired of light and shadow. The 

 greater our knowledge therefore, is, of this subject, 

 the more readily does the illusion seize upon us ; while, 

 if we are but imperfectly acquainted with the effects 

 of light and shadow, the more difficult is it to be de- 

 ceived. If the hollow is not polished, but ground, and 

 the surface round it of uniform colour and smoothness, 

 almost every person, whether young or old, will be 

 subject to the illusion ; but if the object is the raised 

 impression of a seal upon wax, we have often found 

 that, when viewed with the eye-piece, it still seemed 

 raised to the three youngest of six persons, while the 

 three eldest were subject to the deception. By such 

 trifling and often unappreciable circumstances is our 

 judgment affected, that the same person at one mo- 

 ment sees the convexity raised, and at another time 

 depressed, though viewed as nearly as possible and 

 under the same circumstances. This remarkable effect 

 no doubt arises from the introduction of some casual 

 reflected lights, which the slightest change of position 

 will produce. 



Having thus seen haw our judgment concerning 

 elevations and depressions is affected by our degree of 

 knowledge of the effects of light and shade, and by 

 unappreciable causes, we shall proceed to consider how 

 our judgment is affected by the introduction of new 

 circumstances. 



Let the depression A, illuminated by one candle, as 

 in Fig 6, be converted into an elevation as in Fig. 7, by 

 the application of an inverting eye piece; then, if ano- 

 ther candle C', Fig. 8, is introduced so as to illuminate 

 the depression A in the same manner, and with nearly 

 the same intensity as C does, the elevation will fall 

 down into a depression. The cause of this is obvious: 

 the application of the inverting eye-piece produces no 

 effect whatever, for both the sides of the cavity are 

 symmetrically illuminated. In moving round the se- 

 cond candle C' from its position C', so as to stand be- 

 side C, it is curious to observe the progress of the de- 

 ception by which the depression is again changed into 

 an elevation. 



If when the depression A, Fig. 9, is converted into 

 an elevation, we introduce a small unpolished opaque 

 body M, and place it either beside the hollow or in it, 

 so that the body M, and its shadow m, may be dis- 

 tinctly seen by the microscope, we shall have the ap- 

 pearance shown in Fig. 10, the elevation having sunk 

 into a depression. 1'his correction of the deception 

 arises from the introduction of a new illusion, namely, 

 that which arises from the shadow m, for it is evident 

 that as the body M appears, to project its shadow in the 

 direction M m, the luminous body must be supposed to 

 be on the same side; and the evidence that this is the case, 

 is more powerful than our knowledge that the candle 

 is actually at C, because it co-exists along with our 

 perception of the depression A, whereas our knowledge 

 of the situation of the candle is an act of recollection. 



This correction of the delusion may be effected in 

 another manner, which is perhaps more complete. If, 

 in place of the unpolished body, we use a pin with a 

 highly polished head, as shown at M, Fig. 11, and then 

 apply the inverting eye-piece, we shall have the effect 

 shown in Fig. 12, the cavity A appearing depressed. 

 The image s of the candle C being seen by reflection 

 in the polished head of the pin M, is seen by the ap- 



Fig. 8. 



Tig. 9. 



Fig. 10. 



plication of the eye-piece at *, on the right hand side Science, 

 of M in Fig. 12, so that we immediately conceive, in op- Cunpsiitei 

 position to our previous knowledge, that the candle 

 must be at D ; and hence the elevation fails into a ^^Y"*" 

 depression the moment the pin head is pushed up into 

 the field of view. The shadow M m has also its influ- 

 ence in the present case. 



The next case in which this illusion is dispelled, ia> 

 when the sense of touch corrects the deduction formed 

 through the medium of sight. Let the cavity A be 

 raised into an elevation by the inverting eye-piece, as 

 in Fig. 7. Then if the cavity is sufficiently deep, and 

 if we place the point of our finger in the cavity, the 

 evidence which this gives us of its being a depression^ 

 is superior to the evidence of its being a cavity arising 

 from the inversion of the shadow ; the apparent eleva- 

 tion will of course sink into a depression ; but the mo- 

 ment the finger is withdrawn, it will again rise into an 

 elevation. 



Having thus considered some of the principal pheno- 

 mena arising from the inversion of the object, we shall 

 now proceed to explain some analogous facts which are 

 owing to the transparency of the cavities. If M N, 

 Fig. 13, for example, is a plate of mother-of-pearl, and A Fig. 13. 

 a cavity ground or turned in it; then if this cavity is illu- 

 minated by a candle C, or by a window at C, in place 

 of there being a shadow at the side s, as there would have 

 been had the body been opaque, there is a quantity of 

 refracted light seen along the whole side s, next the 

 candle. The consequence of this is, that the cavity 

 appears as an elevation when seen only by the naked 

 eye, as it is only an elevated surface that could have 

 the side s illuminated. The fact which we have now 

 stated, is, we think, a very important one in so far as 

 it may affect the labours of the sculptor. In some 

 kinds of marble, the transparency is so great, that the 

 depressions and elevations m the human face cannot b 

 represented by it with any degree of accuracy ; and> 

 consequently, transparent marble ought never to b 

 used for works of any importance. 



Illusions, arising from the same cause may be ob- 

 served even when the surface of the body is perfectly 

 plain and smooth. If M N, Fig. 14 is the surface of a Fig. It- 

 mahogany table, M N M m a section of it, ami a he a 

 section of a knot in the wood, then it often happens 

 from the transparency of the thin edge at a, next the 

 candle, that that side is illuminated while the opposite 

 side at c is dark, the eye being placed in the plane of 

 the section a be. The consequence of this is, that the 

 spot a b c appears to be a hollow in the table. 



From hence arises the appearance in certain plates 

 of agate, which has obtained for it the name of ham- 

 mered agate. The surface on which these cavities 

 ap.pear is a section of small spherical aggregations of 

 siliceous matter like a be in Fig. 14, wnich present 

 exactly the same phenomenon, arising from the same 

 cause as the knots in mahogany and other woods. 



The very same phenomenon is often seen in mother- 

 of-pearl. Indeed it is so common in this substance, 

 that it is almost impossible to find a ntiother-oi-pearl 

 counter which seems to have its surfaces flat, although 

 they are perfectly so when examined by the touch. 

 Owing to the refraction of the light by the different 

 growths of the shell lying in different planes, the flas- 

 test surface seems to be unequal and undulating. 



One of the finest deceptions which we have ever met 



with, arising from the disposition of light and shadow, 



presented itself on viewing through a telescope the 



surface of a growing field of corn, illuminated by the 



(j 



