6' THE AMERICAN MONTHLY [Jan. 



one insertion of the objective in the draw tube answer for 

 the examination of all the objectives, without its having 

 to be removed for each time. The auxiliary objective is 

 put in position, the one to be examined is put in the nose 

 })iece and its focal distance approximated, which can 

 usually be done pretty nearly by one familiar with the 

 objective. 



While in these experiments no real figures can be got- 

 ten at, it is easily within the power of the maker to 

 sup})ly tliem with high class instruments at a very 

 moderate price. All the other tests of an objective are 

 within easy reach of the worker, why should not this 

 supreme test of its workmanship also be within his reach? 



The principle that the maker can take advantage of is 

 this. The position of the knob which regulates the supply 

 of light through the diaphragm is of course directly re- 

 lative to the size of the opening. 



Fitted over the outer collar of the diaphragm may be 

 attached a plate extending forward two inches, being 

 rounded to an arc of 80° — 90°, with a radius which would 

 be about 3 inches. In place of the knob used to regulate 

 the opening, an index pointer is screwed in place. The 

 arc is so graduated as to indicate the aperture of the ob- 

 jective when the iris diaphragm has been viewed and ar- 

 ranged as above stated. 



While for the ordinary worker the problem of gradu- 

 ating this arc might be very difficult, owing to the fact 

 that very accurate measurements must be made of the 

 diaphragm opening, the refraction of light through two 

 kinds of glass with a spherical triangle of air interposing^ 

 the radius of the part of the condenser used, to be deter- 

 mined, etc., to the practical optician such calculations are 

 easy enough. ^ 



White's Objects. — The Central Board of Kducation, 

 Fifth Avenue Hig-h School Building-, Pittsburg-, Pa., has 

 just purchased 80 White Objects for use of the dei)art- 

 ment of biolog-y, Ed. Rynearson, teacher. 



