40 CONSTRUCTION OF THE MICROSCOPE. 



towards h, and therefore would never arrive at the lens fg, 

 without the interposition of the plano-convex lens at de. 

 placed at a smaller distance from the object ; and by this 

 means the pencil d n, which would have proceeded to h, 

 is refracted or bent towards the lens fg, having a radial 

 point at p q. The object is magnified upon two accounts : 

 first, because if we view the image with the naked eye, it 

 would app&r as much longer than the object as the image 

 is really longer than it, or as the distance fb is greater 

 than the distance from the real object to/' ; and secondly, 

 because this picture is again magnified by the eye-glass. 

 The compound microscope, then, consists of an object-lens, 

 I n, by which the image is formed, enlarged, and inverted ; 

 an amplifying lens, d e, by which the field of view is en- 

 larged, and is consequently called the field-glass ; and an 

 eye-glass or lens fg, by which the eye is permitted to ap- 

 proach very near, and consequently enabled to view the 

 image under a large angle of apparent magnitude. The 

 two, when combined, are termed the eye-piece. 



Mr. Lister's investigations in the year 1829, made for 

 the purpose of improving and correcting the imperfections 

 of the object-glasses of the compound microscope, led to 

 the most important results. Mr. Ross also presented to 

 the Society of Arts, in 1837, a paper on the subject, this 

 was published in the 51st volume of their Transactions: 

 he thus writes : — 



" In the course of a practical investigation, with the 

 view of constructing a combination of lenses for the object- 

 glass of a compound microscope which should be free from 

 the effects of aberration, both for central and oblique 

 pencils of great angle, I obtained the greatest possible 

 distance between the object and object-glass ; for in 

 object-glasses of short focal length, their closeness to the 

 object has been an obstacle in many cases to the use of 

 high magnifying powers, and is a constant source of 

 inconvenience. 



" In the improved combination the diameter is only suf- 

 ficient to admit the proper pencil ; the convex lenses are 

 wrought to an edge, and the concave have only sufficient 

 thickness to support their figure : consequently the com- 

 bination is the thinnest possible, and it follows that there 



