26 SELECTION AND USE 



The Coddiiigton Lens. Whenever a power greater 

 than twenty diameters is required for examining objects, a Cod- 

 dington, if well made, will be found to be the best lens in use, 

 always, of course, excepting the carefully corrected doublets 

 and triplets previously mentioned. The price of the latter, 

 however, is in general four to eight times that of a good Cod- 

 dington. It has this defect, however, that the working focus 

 is very short, and therefore for a dissecting microscope a 

 doublet is to be preferred. In using a Coddingtoii lens, great 

 care must be taken to secure good illumination of the object, 

 and the shortness of the focus makes this difficult to those 

 who have had no experience. 



The Stanhope Lens is similar in form to the Codding- 

 ton, but is very different in construction. It consists of a 

 cylinder or rod of glass, one end of which is rounded so as to 

 form a lens, while the other end is either flat or slightly curved. 

 The distance between the lens and the flat surface is exactly 

 equal to the focal distance of the lens. : Transparent objects, 

 such as the scales of insects, animalculse in water, etc., are sim- 

 ply placed On the flat surface of the glass cylinder, and when 

 looked at through it, they appear greatly magnified. It is 

 easily used, but can not well be employed as a working micro- 

 scope. It is this kind of lens that is used in the construction 

 of those watch charms in which a large picture is seen on look- 

 ing through a very small hole. The picture is a photograph 

 attached to the flat end of a small glass rod, the other end of 

 the rod being formed into a lens of exactly the right focal 

 length required to show the picture clearly and considerably 

 magnified. Lenses and photographs of this kind are usually 

 mounted as miniature opera-glasses. 



Stands for Simple Microscopes. -For ordinary pur- 

 poses of examination, the magnifiers we have just described 

 serve very well when merely held in the hand, but their per- 

 formance is greatly improved when they are mounted on appro- 

 priate stands, which not only enable us to adjust the focus with 

 great accuracy, but which hold the lens steadily in relation to 

 the object, and thus prevent any necessity for that constant ad- 

 justment of the eye itself , which always occurs wiien a lenstrem- 



