SCIENCE. 



i- m , On the Apparent Dtrt( i.ije* in a Portrait. 



A very curious pnper on this subject has been re- 

 'iril In Dr. \Ytiih..;. ,H, in thr I'/tilotr>[>hical 



MMet*d!?' f' rt " ts " ct ' u " J '' f o';in abstract of whidi will he int creating 

 rceti.m ,,t to most of our readers. 



eyts In examining the eyes of a person opposite to us, 



poitruit. anil looking horizontally within a range of about 20 

 on cither .vide of us, we shall find that the white parts 

 oi' tin- eye iiK-ii-.i.^e .mil decrease according as they 

 are turned to or from the no*e. When the eyes are 

 looking straight at us, the two portion! of white are 

 nearly equal, so that by the relntive magnitudes of the 

 white- parts of each eye we can estimate in what de- 

 gree the eyes deviate in direction /rom the face to rvftic/i 

 tiir i/ Li'ltttii;. 



In judging, however, of thnr direction in reference 

 to ourselves, we are not guided by the eyes alone, but 

 by the concurrent position of the entire face. This 

 PLATE M ' i11 be "ndei-tood from Mate C(V( IAXXVI. I- : 



CCCCL.VXXVI. where tne pair of eyes were originally drawn from 

 Fig. 3?, 3. the life by Sir Thomas Lawrtnce, actually looking at 

 him. The face has bet'n added according to the ori- 

 ginal design, so that the person represented in Fig. 2. 

 appeal decidedly looking at the spectator. If, how- 

 ever, a set of features oppositely turned are- applied 

 to the same eyes as in Fig. 3. by lifting up the 

 piece of paper, the eyes will be found to look consi- 

 derably to the right of the person viewing them. 



The same principles apply to instances of moderate 

 incli-nation of the face upwards or downwards ; but 

 the principle is most strikingly exemplified when the 

 turn of a pair of eyes partakes of both inclinations, so 

 as to be in a direction laterally upwards, as in Fig. 4. 

 Fig. t, j. By giving the face a downward cast, as in Fig. 5. the 

 change of effect is very remarkable. Dr. Wollaston 

 considers these examples as proving that the opposite 

 direction of the eyes to or from the spectator, depends 

 on the balance of two circumstances combined in the 

 same representation, viz. 1. The general position of 

 the face presented to the spectator ; and, 2. The turn 

 of the eyes from that position. In the same manner as 

 the general position of the face carries the eyes along 

 with it, so a change in the position of the eyes carries 

 the face along with them. This fact, which is not 

 mentioned by Dr. Wollaston, is not less surprising than 

 its counterpart, and may be well illustrated by causing 

 a pair of invisible eyes to oscillate in the sockets of 

 the eyes of a picture. 



Dr. Wollafcton next proceeds to explain a fact which 

 every person must have observed, that if the eyes of a 

 portrait look al the spectator when he stands in front 

 of the picture, they follow and appear to look at him 

 in every other direction. His explanation and illus- 

 tration of this is every way satisfactory ; but not so 

 popular as we think it may be made. The following 

 illustration appears to us more easily comprehended. 

 If a picture repr.'*ents three soldiers, each firing a 

 musket in parallel directions, and if the musket of the 

 middle one i* pointed accurately to the eye of the 

 spectator, then the muzzle of the musket will be ex- 

 actly circular, and the spectator will see down the 

 barrel, and no part of the right or left side of the bar- 

 rel. In like manner, the spectator will see the left 

 side of the barrel of the soldier opposite his left 

 hand, and the right side of the barrel of the soldier 

 opposite his right hand. If the spectator now changes 

 his place, and takes ever such an oblique position, 

 either laterally or vertically, he must see the same 





thing, became nothing rl*e is painted on the canvas. 'Vckuce, 

 The gun of the middle "tidier mut always point to Cwrfo**" 

 the eye of the spectator, the t- , the 



D,andthegunof thethirfi to the left of him. ^-^ 

 will, therefore, all three *eem to move a* he 

 move*, and follow him in hU motion*. The tame rea- 

 soning is applicable to jwr-pc m-.,. building*. See 

 firewater's Journal nf Science. No. VII. 

 -i. On tin- Optical Illusion of il,>- < W/TTHWI nf C&mtc't 



into Intaglio*, ami of Intaglio* into Cameo*, and other 



Analogous Phenomena. 



At one of the early meeting* of the Royal Society of On the op- 

 London, when a compound microtcope on a 0ew con- **"* *: 

 htruction was exhibited, some of the roembert, while ** 

 looking through it at a guinea, saw the head upon thre ^ n 

 coin depressed, while to other* it appeared to be raised, fotoiaug- 

 as it was in reality. IMM, sad 



The same phenomenon wa* afterward* observed by 

 Dr. P. F. Gmelin of Wurtemberg, while examining 

 objects through telescope* and compound microscope* ; 

 and he seems to have studied it with considerable care, 

 The protuberant parts of objects appeared depressed, 

 and the depressed parts protuberant ; but this hap- 

 pened in some cases and not in others, at some time*, 

 and not at others, and to some eyes and not to othcrt. 

 After a variety of trial* he observed the following 

 constant effects. When he looked at any object which 

 was neither white nor shining, rising upon a plane, 

 with the eye and the optical tube directly opposite to 

 it, the elevated parts appeared depressed, and the dc. 

 pressed part; elevated. Dr. Gmelin is said to have 

 discovered a method of making objects always appear 

 with their natural convexity, viz. by directing hi* sight 

 at first to the edges of the convexity, and then gradual- 

 ly taking in the whole. 



Before we proceed to explain the principles on which 

 this illusion depends, we shall first describe'the best 

 method of observing it. It will afterwards be seen, 

 that telescopes and microscopes- are not necessary to 

 its production ; but it may be best seen by viewing 

 with the eye-piece of 211 achromatic telescope the en- 

 graving upon a seal, when illuminated either by a 

 candle or the window of an apartment. This eye- 

 piece inverts the i.!>ject< to which it fs applied like the 

 compound microscope, and the excavations or depres- 

 sions of the seal are immediately raised up into eleva- 

 tions like a cameo, or a bas-relief. The cause of this 

 illusion will be understood from PlateCCCC I.\ \\ \ I. PLATE 

 Fig. (). where A represents a spherical cavity illumi- CCCCLXXXVI 



i by a candle C. The shadow of the cavity will *HJ- * 

 of course be on the left side S, and therefore if we 

 view it through an inverting eye-piece or microscope, 

 the cavity will be seen as at A, Fig. 7- with its sha- Fig. 7. 

 dow on the right hand S of the cavity. As the candle 

 C remains where it was, the observer instantly con- 

 cludes th;.t what was formerly a cavity must now be 

 a spherical elevation or segment of a sphere, as no- 

 thing but a raised body could have its shadow on the 

 right hand S. If a second candle is now placed on 

 the right hand side of A, so that it is between two 

 candles, and is equally illuminated by both, the ele- 

 vation will again sink into a cavity as in Fig. 6. 



If the object A, in place of being a cavity, is actually 

 the raised segment of a solid sphere, the same pheno- 

 mena will be observed, the inverting eye-piece convert- 

 ing it into a cavity. These two experiments may be 

 made most successfully with a seal, and an impres- 

 sion taken from it. 



It cannot therefore be doubted, that the optical ilia- 



