40 CONSTRUCTION OF THE MICROSCOPE. 



towards h, and therefore would never arrive at the lens/p", 

 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/^, 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 appear as much longer than the object as the image 

 is really longer than it, or as the distance/ 6 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/^, 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 suflScient 

 thickness to support their figure : consequently the com- 

 bination is the thinnest possible, and it follows that there 



