Coscinodiscus asteromphalus. By T. W. Butcher. 729 



ocular 4. Similarly, the upper network or veil lying on the outer 

 surface of the valve is not difficult to display when present, and 

 the photomicrographs shown in October last, taken with a still 

 older 2 mm. apochromatic, though with N. A. 1 * 40, make this 

 quite clear. Indirectly also it is a confirmation of Mr. Merlin's 

 observation in June 1910,* that a similar veil is seen in N. Smithii, 

 and we have already seen lantern slides in which it is present in 

 the two specimens I have photographed. 



I again say, finally, that I believe that this newly described 

 perforated membrane in the place indicated would be an absurdity 

 from a biological point of view. In this case the error of obser- 

 vation minimises considerably the value of the eulogy from an 

 optical point of view that has been passed upon this particular 

 instance of an advance in the manufacture of the apochromatic lens. 



I wish to take this opportunity of thanking Messrs. Watson 

 and Angus very sincerely for the loan of the various stands and lenses 

 through which I have been enabled to exhibit some of the slides 

 mentioned. There are two lenses in use here to-night with which 

 I have photographed these very minute tertiaries, one of them a 

 -| Zeiss apochromat N.A. 1*40, and the other of the same focal 

 length with N. A. 1 ■ 30. There is also a very beautiful lens (3 mm. 

 apochromat N.A. 1*40) set up for me specially by Mr. Powell, 

 with a very perfect specimen under it for examination. 



Conditions. 



Slides mounted by Wm. A. Firth. 



Objective 2 mm. Homogeneous oil-immersion, apochromatic 



N.A. I - 40. Ocular compensating, X 6. Leitz. 

 Tube-length 170 mm. Initial magnification, x 764. 

 Sub-stage condenser : Oil-immersion, N.A. 1 ■ 40. 

 Screen (D + H) : Wratten and Wainwright, M series. 

 Illuminant : Liliput arc lamp. 8 amps. 

 Carbons, cored 8 mm. 

 Auxiliary condenser with iris. 

 Backed plate : ordinary. Speed 225 (H. and D.). 

 Developer : Metol hydrokinone. 

 Average exposure : 12 sees. 



See this Journal, 1909, p. 399 ; and Journ. Quekett Micr. Club, 1908, pp. 247-50 



