J'llE PliESIDEXT's ADDKESS. 187 



yet it is certain that comparatively few microscopists realise how 

 much there is to learn before full advantage can be derived from 

 its employment. To prove this I may be permitted to recount an 

 item of personal experience ; premising that I have used the 

 accessory in question continuously for a period of at least 

 twenty-five years. 



One evening, only a few months since, I was examining, with 

 Mr. Nelson's assistance, a new and very perfect apochromatic 

 condenser recently constructed for me by Messrs. Powell & Lealand. 

 Finding that on examining a certain test slide, which was a 

 very thin one, I was unable to obtain an unbroken cone of light 

 nearly large enough to fill the back lens of an objective of 

 the same numerical aperture as that of the condenser itself, I 

 hastily came to the conclusion that the condenser was at fault, 

 and resolved to return it to the makers for alteration. From 

 his riper knowledge, however, my host was able to teach me that 

 the fault lay — not in the condenser, but in myself. Under his 

 direction I learnt to appreciate full}^, for the first time, the fact 

 that no condenser can be adjusted to give a perfect cone with 

 more than one definite thickness of slip. And this was shown to 

 be the case with the condenser in question, for on changing the 

 glass slip, which was perhaps less than ^\j^-inch thick, for one of 

 double that thickness, the back lens of my objective, an apochro- 

 matic of 0-95 N.A., was instantly filled with an aplanatic cone of 

 bright light, and the resolution of the test was thenceforth 

 accomplished with ease. The practical lesson, which this single 

 experience has taught me, is that good resolution and perfect 

 definition can be obtained, with even the most perfect appliances, 

 only when glass slips are used which are of the proper thickness, 

 and which approximately fill up the space intervening between 

 the front lens of the condenser and its focal point ; and as the 

 object under examination is, or should always be, in the exact 

 focus of the condenser, it is easy to arrange the thickness of the 

 glass slips accordingly. When difficult or exceptionally trans- 

 parent diatoms are under examination the correctness of the 

 foregoing contention readily becomes apparent. For example, 

 I have very recently been examining a specimen of plei(TOsigma 

 maci'um, mounted in Styrax, on a very thin slip. Forgetting 

 the experience already referred to, I spent a considerable part of 

 one evening in fruitless attempts to obtain resolution of this 



