ing to have the same refractive index, or density, 

 as glass, the two by contact become virtually homo- 

 geneous, and no refraction takes place; the rays 

 do not stray away, as they would in air, but are 

 passed without loss direct through the lens. 



In passing, let me point out, there is no produc- 

 tion of the human hand that is more delicate, none 

 which must be so faultless as the objective 

 lenses of the compound microscope. In the finer 

 processes of their manufacture, all mechanical 

 measurements fail, and recourse must be had to 

 highly sensitive optical tests to determine errors. 



And here again our diatom deserves distinction 

 of a kind. For it has been of inestimable aid to lens 

 makers in the perfection they have already 

 achieved; it is the minute markings of certain 

 species — among which those of P. angulatum serve 

 exceedingly well — that they use to test the prop- 

 erties of their lenses. 



If I have gone into this subject to some extent, 

 it is because it would seem that, aside from spe- 

 cialists and others who regularly use this instru- 

 ment in their work, there exists a general ignorance 

 regarding its capacity and possibilities for common 

 enjoyment. It is quite probable that if the micro- 



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