34 SELECTION AND USE ' 



vided with a handle, and qiter a drop of water has been placed 

 on the flat surface of the lens, the cap, c, is screwed on, and the 

 object examined by simply holding the instrument up to the 

 light. 



A very excellent collector's microscope will also be found de- 

 scribed amongst the compound instruments. 



The Coddington Lens. This lens was devised by Sir 

 David Brewster, but having been made by a London optician 

 for Mr. Coddington, it was called by his name, which has stuck 

 to it ever since. It consists of a cylinder of glass, the two ends 

 of which have been ground so as to form portions of the surface 

 of the same sphere. A deep groove is cut around the cylinder, 

 midway between the ends, and a diaphragm is thus formed 

 between the two lenses. In Figure 9 is shown a very neat 

 and convenient method of mounting the Coddington. 



This form of lens gives very sharp definition, so that when- 

 ever a power greater than twenty diameters is required for 

 examining objects, a Coddington, if well made, will be found to 



be the best lens in use, always, of 

 course, excepting the carefully 

 corected doublets and triplets 

 hereafter described. The price 

 of the latter, however, is in 

 general four to eight times that 

 of a good Coddington. But it 

 has this defect, that the working 

 Fig. 9. CODDINGTON LENS. focus is very short, and therefore 

 for a dissecting microscope a 



doublet is to be preferred. In using a Coddington 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. 4* 



Those who desire to acquaint theinselvas with the structure 

 and peculiarities of the most important simple microscopes, 

 will find this subject very fully and very clearly treated in the 

 article contributed by Andrew Ross to the " Penny Cyclopaedia," 

 published by the Society for the Diffusion of Useful Knowl- 

 edge. This article has been republished in book form. 



