NATURAL PHILOSOPHY. 125 



their proper tint, and the whole thing has a singularly vital and comfort- 

 able look. Indeed, were it not for the reduction in size, it would be 

 difficult to avoid the belief that an actual man or woman, in ordinary 

 dress, and with characteristic expression, was presented to your eye. 



This curious and beautiful effect is produced ~by a new application 

 of the principles of binocular vision employed in the ordi nary ^ stereo- 

 scope. Two portraits (taken at an angle suitable -for the effect intend- 

 ed) are produced by the ordinary photographic means. To effect the 

 combination of these, the block of glass'or quadrangular prism, in the 

 interior of which the solid image is to appear, is composed of two rect- 

 angular prisms ground to an "angle of about 39 or 40. These are 

 placed together so as to form one solid quadrangular prism, divided 

 lengthwise by a thin film of air. If one of the pictures be now placed 

 at the back of this combination, and the other picture at the side, on 

 attempting to look through the combination the two images will be su- 

 perposed on each other (forming one solid image, apparently imbedded 

 in the crystal), all the rays which fall on one side of a line perpendicu- 

 lar to the surface of the prism next the eye suffei*ng total reflection at 

 the inner oblique surface of that prism, while nearly all those rays 

 which fall on the other side of this line will be transmitted, unaltered 

 in direction, through the body of the combination. Thus one of the 

 eyes only perceives the object at the back at the prisms, while to the 

 other eye the picture at the side is alone visible, and that lying appar- 

 ently at the back also, producing the perfect appearance of solidity. 

 It is evident that, to produce these results, care must be taken, not 

 only that the pictures are not misplaced so as to produce the pseudo- 

 scopic effect, but also that the picture which suffers reflection shall be 

 reverted to compensate for the reversion occasioned in reflection. 



All these portraits are viewed as transparencies ; the photographs 

 being printed from ordinary negatives on small mica plates which are 

 affixed to the prisms. 



THE VELOCITY OF LIGHT AND THE SUN'S DISTANCE. 



From an article contributed to the American Journal of Science (Sept. 

 1863), by Prof. Levering, of Cambridge, we derived the following in- 

 teresting memoranda respecting the above subjects : Four methods 

 have been devised for determining the velocity of light, two of which 

 may be termed astronomical and two experimental. The results ob- 

 tained from these determinations, although agreeing so essentially as to 

 prove their comparative accuracy, yet differ slightly among themselves 

 to an annoying and at present inexplicable extent. The first discov- 

 ered method for obtaining the velocity of light, was by observations on 

 the eclipses of Jupiter's Satellites, and the result obtained was 193,- 

 350 statute miles per second. The second process which astronomy 

 has supplied is through observations on the aberration of light, and 

 the result obtained gives 191,513 miles as the velocity of light per sec- 

 ond. The determination of the velocity of light, by the two methods 

 of astronomy, differ therefore by 1837 miles ; a small quantity compar- 

 atively, being only 1 per cent, of the whole velocity. It should be also 

 stated that the velocity which aberration ascribes to light belongs to it 

 at the earth's surface ; that is, in the dense atmosphere ; whereas the 

 velocity discovered from the eclipses is that which extends from the 



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