178 HOW TO SEE WITH THE MICROSCOPE. 



other.) We found at once, that in order to obtain suf- 

 ficient light, and retain the general vigor of the image, 

 it was necessary to approach the radial bar to the axis 

 and the required movement of the latter was quite per- 

 ceptible. It was therefore accepted that the one-sixth 

 had the higher balsam angle. The question then turned, 

 -as a matter of course, on the respective working dis- 

 tances; that of the one-sixth was known. It remained, 

 therefore, simply to measure that of the one-fourth, 

 resulting as follows : The working distance of the one- 

 sixth is twenty-four-thousandths of an inch; while that 

 of the one-fourth was found to be thirty-two-thou- 

 saudths of an inch, a difference of 33 per cent in favor 

 of the one-fourth. Thus it will be seen that in this 

 instance the question as to superiority may be further 

 taken under advisement. We relate this bit of " prac- 

 tice" with the illuminator in illustration of the pre- 

 ceding remarks. 



One thing was proven even by the above experiment, 

 to wit: Having our tenth on hand, held in reserve for 

 especial demands, we would greatly prefer the one- 

 fourth as an intermediate oflass. This fact is too obvious 

 to need further comment, and, in general, we are glad to 

 add that the new one-fourth of the Messrs. Spencer is 

 indeed a lovely glass, and if properly exhibited in Paris, 

 will be an honor to the talented makers. 



The modified illuminator above described (obtuse 

 angle, 98, acute angles, 41 each) will work very well 

 with objectives having wide air apertures only; hence, 

 like the modified " reflex," it will work over dry mounts, 



