T. F. SMITH ON ARACHNOIDISCUS. 253 



apertures without increasing the aberrations, or decreasing the 

 working distance, and without increasing the price. They do not 

 claim that the same results could not be produced by the old 

 glass, but only after a long working-up, and at a price which 

 would preclude them from being sold as student's lenses. 



Personally I do not believe that the same level was ever reached 

 with the old dry glasses, and the crispness of image in the new, 

 which is only equalled by the old water-immersions, is entirely due 

 to the new glass. 



The general lesson I wish to inculcate then is, that knowing 

 what is possible, microscopists should not be satisfied with less 

 than the most absolute perfection in all modern objectives, and that 

 they are not likely to get it unless they insist upon their being 

 made partly at least with the new glass. It is for this purpose I 

 beg to introduce this new test, and I can promise that any glass 

 which will work on this will do whatever else a dry glass can do. 



