ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



509 



the centre of the top of the apparatus, are placed two other prisms, 

 which reflect upwards the rays which they receive from the first pair 

 of prisms. This comparison-chamber is fixed on two Microscopes 

 without eye-pieces, and an eye-piece is placed above the central prisms. 

 By these means I obtain 



a circular field of view Fig. 95. 



composed of two halves, 

 divided by a fine line ; one 

 half belonging to the image 

 from the first Microscope, 

 the other to that from 

 the second. If now two 

 minerals absolutely identi- 

 cal in colour and lustre 

 are placed under the two 

 Microscopes there will 

 appear in the eye-piece 

 of the chamber a com- 

 pletely uniform image, so 

 that the line of division 

 disappears. The slightest 

 change in the tint of one 

 of the objects causes the 

 sudden reappearance of this line, the image being again divided into 

 two distinct parts. 



I think I am justified in supposing that my comparer may be 

 applied not only to the study of minerals and rocks, but equally to 

 all microscopic researches in which comparison is employed. 



To bring out better the colour and lustre of the minerals, I 

 illuminate them by means of small mirrors placed on the stage of the 

 Microscope. For an account of the construction of these, and of the 

 scale of comparison, I must refer to a detailed account which will 

 shortly be published. I may add that in my scale I replace the 

 natural opaque minerals, which would themselves be too expensive, 

 by artificial colours prepared from the powder of these minerals. 

 Under the Microscope the effect is precisely the same." 



Astigmatic Eye-piece.* — Mr. E. Gundlach criticizes Dr. J. K. 

 Stockwell's criticism f of his proposed astigmatic eye-pieces, and 

 considers that the latter's suggestion of cylindrical lenses in spectacle- 

 frames is objectionable on the ground that spectacles should never be 

 used with any optical instrument, as they are always injurious to its 

 proper performance, and, therefore, the wearer of spectacles should 

 always remove them before using the Microscope or telescope. 



That spectacles are injurious is attributable mainly to the 

 following reasons : In the first place they prevent the eye reaching 

 its proper place, in proximity, to the eye-piece. Secondly, the 

 generally very eccentric and oblique position of the spectacle-glass 

 to the optical axis of the eye, and, consequently, also of the instru- 



* The Microscope, vi. (1886) pp. 63-5. 



t See this Journal, ante, p. 313. 



