146 YEAB-BOOK OF FACTS. 



formed by the thinnest achromatic lens, of such a focal length that 

 the object may be an inch or more from the lens, and its image 

 equal to, or greater or less than, the object. In this way the ob- 

 server will be able to illuminate the object, whether opaque or trans- 

 parent, and may subject it to any experiments he may desire to 

 make upon it. It may thus be studied without a covering of glass, 

 and when its parts are developed by immersion in a fluid. 3. The 

 sources of error arising from the want of perfect polish and perfect 

 homogeneity of the glass of which the lenses are composed, are, 

 to some extent, hypothetical ; but there are reasons for believing — 

 and these reasons corroborated by facts — that a body whose in- 

 gredients are united by fusion, and kept in a state of constraint 

 from which they are striving to get free, cannot possess that homo- 

 geneity of structure, or that perfection of polish, which will allow 

 the rays of light to be refracted and transmitted without injurious 

 modification. If glass is to be used for the lenses of microscopes, 

 long and careful annealing should be adopted, and the polishing pro- 

 cess should be continued long after it appears perfect to the optician. 

 Sir David believes that the time is not distant when transparent 

 minerals, in which their elements are united in definite proportions, 

 will be substituted for glass. Diamond, topaz, and rock-crystal 

 are those which appear best suited for lenses. The white topaz of 

 New Holland is particularly fitted for optical purposes, as its double 

 refractions may be removed by cutting it in plates perpendicular to 

 one of its optical axes. In rock crystal, the structure is, generally 

 speaking, less perfect along the axis of double refraction than in 

 any other direction, but this imperfection does not exist in topaz. 



ANTIQUITY OF THE STEEEOSCOPE. 



At the meeting of the Photographic Society of Scotland, Sir 

 David Brewster, President, has read a paper of historic interest, 

 entitled, "Notice respecting the Invention of the Stereoscope 

 in the Sixteenth Century, and of Binocular Drawings by Jacopo da 

 Empoli, a Florentine artist." Sir David said that, inquiring into 

 the' history of the stereoscope, he found that its fundamental prin- 

 ciple was known even to Euclid ; that it was distinctly described by 

 Galen 1500 years ago ; and that Baptista Porta had, in 1599, given 

 such a complete drawing of the two separate picture* a* .*. on by each 

 eye, and of the combined picture placed between them, in which 

 we recognise not only the principle, but the construction of the 

 stereoscope. Last summer, Dr. John Brown, while visiting the 

 Musee wicar at Lille, observed two drawings placed Bide by side, 

 and perfectly similar. These drawings were by Jacopo Chimenti da 

 Empoli, a painter of the Florentine school, who was horn in 1554, 

 ami died in 1840. They represent the same object from points of 

 view slightly diiferent. That on the right hand is from a point of 

 vi.-w slightly to the left of that on the left hand. By com erging the 

 optic axes, the piotures could !»■ united i o at to produce an ini 

 relief, 01 easily and as perfectly as with an ordinary stereograph. 



