6 THK AMERICAN MONTHLY [Jan. 



one insertion ottlie objective in tiie draw tube answer for 

 the examination of all the objectives, without its having 

 to be removed for ea.('h time. The auxiliary ubjective is 

 put in [lositiou. the one to be examined is put in the nose 

 piece and its focal distance approximated. \vhi«;h can 

 usually be done pretty nearly by one familiar with the 

 objective. 



While in these experinuMits no real figures can be got- 

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

 snp|»ly them with liigh class instruments at a very 

 moderate price. All the other tests of an objective are 

 within easy reach of tlie worker, why should not this 

 supreme test of its workmanship also be within his reach? 



The principle that the maker can lake advantage of is 

 this. The j)()sition of the knob which regulates the supply 

 ttf 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 oi)euing, an index [toiuter is screwed in i)lace. Tiie 

 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 ordinai-y witrker the j>rol.lem of gradu- 

 ating; this arc might be very ditticnlt, owing to the fact 

 that very accurate measurements must be made of the 

 diaphragm opening, the refraction of light tlirougli two 

 kinds of glass with a spherical triangle of air interposing 

 the radius of the [tarl of the condenser used, to be deter- 

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

 easy enough. 



White's Objects. — 'l^he Central lloaiil of Kducation, 

 Kifth Avenue Hij^-li School iiuilding. l'ittsl)ur<j-, Pa., has 

 just jnirchased So White Objects for use of the «lepai-t- 

 ment of l)iol(,yv. Kd. Kyneai'son, teacher, 



