MAGNIFYING POWERS. 



ler 



E 



-— '-'V 



toatic object-glasB, in connection with the eye-piece, E E, 

 F F, the planoconvex lens, E E, being the eye-glass, and F F 

 the field-glass, and between them at B B 

 a dark stop or diaphragm. The course 

 of the light is shown by three rays, 

 drawn from the centre, and three from 

 each end of the object, o ; these rays, if 



A.^ tt/- tN- w-./-_ not prevented by the lens, F F, or the 



j diaphragm at B B, would form an image 

 I at A A ; but as they meet with the lens, 

 F F, in their passage, they are con- 

 f verged by it, and meet at B B, where 

 the diaphragm is placed to intercept all 

 the light except that required for the 

 formation of a perfect image, the image 

 F at B B is further magnified by the lens, 

 E E, as if it were an original object iu: 

 the manner described at pages 67 and 

 163. The triple achromatic combination 

 constructed on Mr. Lister's improved 

 plan, although capable of transmitting 

 large angular pencils, and corrected as 

 to its own errors of spherical and chro- 

 matic aberration, would, nevertheless, 

 be incomplete without an eye-piece of 

 peculiar construction. As this subject 

 has been so admirably treated by Mr. 

 Koss, in the Penny CyclopcBdia, it has 

 been thought most desirable to quote 

 his own words, as follows : — 



" If we stopped here," says Mr. 

 Eoss, "we should convey a very im- 

 perfect idea of the beautiftd series of 

 corrections effected by the eye-piece, 

 and which were first pointed out in 

 detail in a paper on the subject, pub- 

 lished by Mr. Varley, in the fifty-first 

 i'"iff. 117. volume of the Transactions of the So- 



