SELECTION OF COVERING GLASS. 213 



to-day, then will the superiority of such glasses be 

 demonstrated, and for reasons already given, and we 

 are disposed to look in this direction for future improve- 

 ments. Let the reader also remember that the item 

 of " working distance" must not be lost sight of. For 

 instance, if, in the extension of the one-fourth to the 

 capacity of the present one-tenth, the working distance 

 of the former should be decreased to that of the latter ; 

 then the real gain would be comparatively slight, 

 amounting practically to a saving of a few dollars in 

 the cost of the glass. The true problem is to gain 

 working force and efficiency without sacrifice of work- 

 ing distance. We recognize what has been accom- 

 plished, and the recognition is suggestive of further 

 advances in the same direction. 



SELECTION OF COVERING GLASS. 



This can be readily obtained of the dealers either in 

 sheets or ready cut into squares or circles, and of any 

 desirable thickness. As to the mere form of the cover 

 used, that is a matter of fancy; but the thickness ought 

 to ccrrespond to the working of the objective. We 

 try to confine ourselves to three thicknesses of cover- 

 ing glass, namely, one-seventieth, one-one-hundred-and- 

 twentieth and one-two-hundredth of an inch. These 

 may respectively be denominated as thick, medium and 

 thin. It is a matter of the first importance that those 

 working first-class objectives should be well posted 

 as to the thickness of cover employed, and yet this tell- 

 ing point has been utterly lost sight of in the books. 



