A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 207 



increased blackness ; the only lens tried with which no actual 

 improvement was noticeable being the 3-mm. (g-in.) Zeiss apochro- 

 mat of IsT.A. 1*425, and even in this instance it proved almost a 

 " tie," only an extremely slight advantage appearing to rest with 

 the compensating ocular. This fact is of great importance to the 

 microscopist of limited means, for the new eyepiece, being simpler, 

 should be less expensive than the complicated cemented forms, 

 and a set of two (one x 24 having been lately computed) would 

 be amply sufficient for the general worker, and could be equally 

 advantageously employed for different kinds of objectives as 

 yielding the best yet attained results with them all. The 

 ■construction of this eyepiece is open to all opticians, the full 

 formula having been published.* 



The next point relates to an objective of the utmost general 

 utility, manufactured by Zeiss for some years past, but not 

 catalogued by that firm, owing apparently to the small existing 

 demand for it. It is a 36-mm. (l|-in.) semi-apochromat of the 

 highest class. A recently -purchased specimen of this lens in my 

 possession has a measured N.A. of '21, and the perfection of its 

 corrections is such that it has even visually rivalled in per- 

 formance an expensive fluorite apochromat of the same power, 

 while its superiority to ordinary l|-in. objectives is unmistakable. 

 One would suppose this to be quite an ideal lens for the naturalist 

 and hundreds of other kindred low-power workers, especially as it 

 is not costly. t Yet, although it has been procurable for upwards 

 of three years, the consecutive number borne by my own objective, 

 obtained a few months ago, is only 20 ! 



Lastly, I would emphasise the desirableness of employing 

 mounting slips of one definite thickness for both low and high 

 power objects, as the beautiful aplanatic sub-stage condensers now 

 procurable from our opticians are sensitive to any pronounced 

 differences in the thickness of the glass ^they work through, and 

 will not afibrd large unbroken solid cones when used under 

 unsuitable conditions. On trial it will be found that most 

 modern dry condensers possess ample working distance and 

 perform most perfectly through moderately thick slips ; these 

 should consequently be accorded the preference by the preparers 



* Carpenter's " The Microscope," 8th edition, p. 377. 

 t Its price is 3os. 



