156 THE AMERICAN MONTHLY [May, 



glass and, when possible, in some medium to diminish loss 

 of light by reflection. As to objectives I must say that the 

 best lens of about an inch focus tried by me is a seven- 

 eighths bought of Queen & Co. It gives a bright field about 

 eight feet in diameter, without any substage condenser, much 

 brighter than does any lens I have tested with or without an 

 accessory, and it bears a one-and-one-half inch (Bj eye piece 

 well when used with a substage condenser. Of the higher 

 powers several makes of about one half and one quarter inch 

 give good results. The best, so far, in my experience are a one- 

 half (adjustable) of Tolles. and a one-quarter (professional) of 

 Bausch & Lomb. The analyser is a Nicol prism about three- 

 fourths of an inch across the face. 



This cuts off some of the field on the screen, but that matters 

 little with "dark field." 



Now let me give results, first describing the conditions. The 

 room measures thirty nine and a half by twenty four and a half 

 feet, and is rather dimly lighted. The distance from micro- 

 scope to screen is thirty one and a half feet. The magnifying 

 powers of the extreme powers used, as determined by project- 

 ing a stage micrometer, are three hundred and eighty four 

 diameters with the seven eighths, and fifteen hundred and sev- 

 enty with the quarter. With the lower power most of the de- 

 tails of sections of granite, and of some other objects of coarse 

 structure, can be seen without turning down ihe lights. In 

 darkness the image is very bright and distinct. With the 

 higher power, and darkened room, nearly all the smaller polar 

 objects are satisf;ictorily seen from any point in the room. To 

 specify by a well known object; the larger tons les mois starch 

 grains, (glycerine-jelly mount) are clearly shown with the 

 characteristic black cross. Dichroism and stauroscopic effects 

 not demanding extreme angle of beam, are completely exhib- 

 ited 



Whether these results have been surpassed elsewhere we do 

 not know, and it is hoped that information as to achievements 

 and appliances will be elicited by this communication. — F. C, 

 Van Dyck. 



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