196 Transactions of the Society. 



petent, and patient pursuit of the study with our always improving 

 optical resources. At least, small as my contribution may be, I 

 am glad to add it to what I think so important an investigation. 



Those of our Fellows who can in any way recall my successive 

 work on the Hfe-histories of certain monads — more or less relatively 

 large septic organisms — will remember that from 1873 and onwards, 

 I, in connection with my friend Dr. Drysdale, called attention to and 

 described and figured the division of the nucleus belonging to these 

 simple living forms. As our work progressed, nuclear division 

 became a more and more noticeable and interesting factor. While 

 in the two latest forms which I studied alone, the history of the 

 former of which was read before the Eoyal Society in 1878, some 

 very suggestive details were observed, which are to some extent 

 indicated in the drawings accompanying that paper. But they 

 were relegated for future and further examination, and in the well- 

 founded hope of the production of finer lenses than we even then 

 possessed. And it is to the immensely improved lenses which we 

 now possess that my later results are mainly due. One may be fain 

 to confess a weariness of reiterated comparisons of lenses based 

 upon delicate diatomaceous striation or " dotting." At best, mani- 

 pulation and the " personal equation" enter largely into the results. 

 But it is not thus in the high-power observations with which for 

 so many years I have been chiefly concerned. With lenses con- 

 structed from fifteen to ten years ago, I worked during those 

 years with definite results. The lenses were the best that the 

 science and art of the time could produce ; and the organisms on 

 which the researches were made were thoroughly known, and were 

 examined through consecutive years under every variety of con- 

 dition, optical and other ; while the limits of disclosure were clearly 

 known, and can be readily shown with the same lenses on the same 

 objects to-day. 



But during the following ten years, bringing us to the present 

 time, there have been, as this Society has so efficiently shown, 

 splendid improvements in our lenses. 



It would divert our attention needlessly for me to attempt an 

 historical account of these steady advances. Each year has had 

 its optical trophy, English, German, or American. The highest 

 and latest class of water-immersions, as made by Powell and Lealand, 

 proved a large gain over all their predecessors in searching into 

 minute living structure, and delicate organic changes ; and their 

 possession soon convinced me that by a further extension of what 

 we now call N.A., much more was to be discovered in relation to 

 these simplest and minutest organisms. Then followed the early 

 homogeneous lenses of Abbe and Carl Zeiss, which showed great 

 advance in the direction needed, and still greater potentiaHty and 

 promise. What was required was the increase in these lenses of 



