Minute Organic Particles. 25 



Other splendid instances of sharpness of definition may almost 

 always be seen by using the very lowest eye-piece obtainable, with 

 a very excellent objective, such as a good ^^^th immersion. In 

 order to use my -^th more conveniently, and to remove it somewhat 

 from its dangerous proximity to the object, I have had a body made 

 so as to reduce the conjugate focus one-half. The definition, after 

 correction, is one of the most charming effects I have ever seen, 

 especially with an eye-piece of long focus as 3 inches. 



As an instance of the beautiful precision of this objective (under 

 favourable circumstances), I will state one experience with it. 



I was examining a Poclura eurvicollis. The sun was setting. 

 A fine greenish-blue sky coloured the west. A plane mirror. The 

 Boss 1^ was in use as a condenser, unstopped ofi", and perfectly 

 direct without obliquity. The entire field was filled with the ribs 

 of the Podura. To my great delight, the whole of the white spaces, 

 generally seen absolutely blank, appeared crowded with white beads 

 in long rows, as shown within the ring drawn within Fig. 3. 



If perfect definition is now required, microscopists must take a 

 new start and destroy residuary errors by preventing the admission 

 of obstinately aberrating rays : and using monochromatic or unique* 

 rays (as Dr. Col. Woodward in his splendid photography). 



I shall conclude this imperfect paper by pointing out the gradual, 

 infantine, growth of magnificent microscopic defining power. 



Lines preceded spherules. 



In the spaces, betiveen the coarse markings of the Podura 

 Degeeria, short black cross lines were at first delineated (Mr. 

 Mclntire, 'M. M. J.,^ Jan. 1870, Fig. 6, Plate XXXYII.). Next Col. 

 Woodward photographed shadowy bars (see Fig. 2, Plate XXIII.). 



Lined objects gave place to beaded (scales and diatoms). 



Black dots of Quekett gave way to spherules. Nobert himself 

 declared his XlXth Band would never be resolved, and he purposely 

 concealed the precise number of the lines in it, but awarded the palm 

 to Dr. Colonel Woodward, who photographed them with Powell and 

 Lealand's sixteenth immersion and copper solution for a unique 

 light of blue rays. 



The same lens succeeded in turning the misty bars of the clear 

 spaces between the markings of the Degeeria into beads, and the 

 supposed blank spaces of the True Test Podura into similar rouleaus. 

 And I here gratefully acknowledge the favour of their photographs 

 being placed at my disposal by their courteous and distinguished 

 author. 



Postscript. — Professor Stokes, Sec. E. S., 'Br. Ass. Eeports,' 

 1871, has communicated the very important fact that titanic glass 

 has the property of destroying the secondary spectrum : and a ter- 

 * Unique seems a more simple term than the long Greek compound. 



