308 Dallinger, Drysdale's u. Dallinger's Untersuchungen an Biflagellatett. 



corrected the essential defects of these lenses. Their power was always 

 too great for their numerical aperture. But this could only be got 

 over in one way, viz the employment of a homogeneous medium which 

 would make large numerical aperture possible; and the use of some 

 new ,,optical metal" or glass which should have higher refractive and 

 dispersive indices. At the suggestion of Mr. Ware Stephenson Prof. 

 Abbe gave a practical form to a homogeneous system of lenses, 

 enormously benefiting microscopy; and by the combined work know- 

 ledge and ingenuity of Prof. Abbe, Dr. Schott and others new optical 

 media were made, which gave a new value to and formed an epoch 

 in the manufacture and use of the modern microscopes. How deeply 

 we entered into and appreciated these optical improvements as aids 

 to investigation it is enough for me to appeal to the Proc. of the 

 Royal Microscopical Society, or to my edition of the late W. B. Car- 

 penter's ,,The ,,Revelations of the Microscope" to establish. 



For ,,critical images" and final certainty on any delicate question 

 of the nature of a microscopical image these apochromatic lenses of 

 Abbe are indispensible. 



Nevertheless, not only before, but after the introduction of these, 

 the over- amplification of the series of achromatic lenses I have des- 

 cribed above was equally iudispensible. The minute organisms studied, 

 always in an active state, could never have had the details of their 

 life-cycles made out with immersion lenses of any kind -- water or 

 oil. We required great enlargement; but with dry lenses; for in 

 following the constantly moving organism, so as to keep it conti- 

 nuously in sight, we were obliged to keep the mechanical stage in 

 constant action, not only in rectangular but in diagonal movement; 

 of which the beautiful stage of Powell and Lealand has for nearly 

 half a century allowed. 



But by such necessary motion with the delicate continuous gro- 

 wing stage" employed in our work, it becomes clear to the practical 

 worker, that at length, in spite of any amount of care, the water 

 employed for ,,immersion" between the front lens and the cover-glass 

 would ultemately pass the edge and mingle with the fluid in which 

 the living organism was being studied; and so destroy the observation. 

 This would be still more inevitable if the immersion fluid were oil. 



Clearly then dry object-glasses with great magnifying power, 

 illuminated by a suitable condenser were essential to our method, 

 and even to-day must be employed on such observations. 



But this by no means prevented the use of the new apochromatic 

 objectives; they became a valuable supplinient, and admirable mode 

 of testing the work we had hitherto done: and after the homogeneous 

 objectives were introduced, and again after the apochromatic object- 

 glasses were in our hands, we went over every important point - - not 



