110 THE MICROSCOPE. 



PRACTICAL SUGGESTIONS. 



Rapid Polarization — Since writing my last article on this 

 subject (JDruggisVs Circular, June,) I have simplified the pro- 

 cedure still more, so as to make it more convenient. My mod- 

 ification Ccin be applied to any microscope provided with a dia- 

 phragm wheel. Solder to the largest hole of the diaphragm a 

 short tube, say three-quarters of an inch long, and fit into this 

 tube a nicol, turning the latter so that the long diameter runs 

 from front to back, when it is brought under the hole in the 

 stage; the nicol is then in the best position. ( An inspection of 

 a mounted nicol, polarizer or analyzer, will show that the nicol 

 is held in place by pieces of cork.) In polarizing, I merely re- 

 volve the diaphragm so that the nicol comes under the hole in 

 the stage, and then clap on the tourmaline (or other nicol) over 

 the eye-piece. There is then no need to be careful to face the 

 light, nor to observe any particular slope of the microscope 

 tube, and, what is better still, the illumination is a good deal 

 brighter than with a black glass. If there is no diaphragm 

 wheel, recourse must be had to the black glass, the illumination 

 of which may be greatly improved, however, by super-imposing 

 a plate of ordinary glass. The optical reasons for this improve- 

 ment are somewhat obscure, a thick piece of plate glass, black- 

 ened on the reverse side, does not brighten the illumination as 

 much as a piece of ordinary glass on top of a thin, blackened 

 glass. Still easier, place a piece of glass (a slide, for instance) 

 on top of a 'piece of black u japanned v tin. — H. M. Wilder. 



CORRESPONDENCE. 



THE TARIFF ON BOOKS. 



In general I am opposed to any tariff except for revenue. I 

 think I have good grounds for that position, but a discussion 

 of them would lead too far at this time. On some articles I 

 am opposed to any tariff whatever. Among the latter are scien 

 tific books and educational works in general. 



My reasons for so thinking are briefly stated as follows: 



