100 E. M. NELSON ON PLEUROSIGMA ANGULATUM. 



on the structure of the genus Pleurosigma, because this note, 

 not being an exposition of the structure of this genus, deals 

 merely with the single fact of my altered opinion with regard 

 to the apertures in the lower membrane not " eye-spotting " 

 those in the upper. 



The genus Pleurosigma has been seventy years before the 

 microscopical world, not laid aside, but worked at continuously 

 by the most skilful microscopists, yet all the problems con- 

 nected with their structure have not been solved. It is only 

 by recording from time to time a little bit here and a little 

 bit there, and by putting these little bits together, that complete 

 and accurate knowledge of this difficult subject will be attained. 



ACTINOCYCLUS RALFSII AND A COLOURED COMA. 



By E. M. Nelson, F.R.M.S. 

 {Read January 28th, 1913.) 



The following account of a microscopical phenomenon, never 

 previously observed, may be of interest. 



When working on a mixed diatom gathering, dry and un- 

 covered, with a Powell & Lealand | inch and a lieberkuhn, there 

 appeared round an Actinocyclus Ralfsii a wide border of brilliant 

 orange, green and blue light. The inside of the valve was 

 uppermost and the bottom of the cup was in focus, so that the 

 surrounding mist was caused by the out-of -focus edge, which, of 

 course, was at a higher level. Any one seeing this coloured mist 

 would have exclaimed what a badly corrected objective ! but if 

 they had looked at the other diatoms in the field, they would 

 have seen that the out-of -focus coma was white ! The colour, 

 then, must be a function of the Actinocyclus. Another objective 

 with a lieberkuhn, viz. a Powell & Lealand T 4 ^ inch, was tried on 

 the same diatom ; the border was now red, the green and blue 

 having gone ! A third objective, viz. a Wenham | inch with 

 lieberkuhn (really a T 4 <y inch), and the image seen had no colour ! 



Here, then, we have an example of an object affecting different 

 objectives differently. Would some of our " brass and glass " 

 experts kindly take this matter up ? 



Joara. Quekett Microscopical Club, Ser. 2, Vol. XII., No. 72, April 1913. 



