Microscopic images with high powers. 257 



must of necessity be a truer picture than that which shows 

 them all as being precisely similar. I was very much struck 

 on first noticing this differentiation. I carefully noted the 

 valve, and the part of the valve, where it occurred. I have 

 spent, now, upwards of five years working at this same spot, to 

 see if I could get any elucidation as to the cause of these 

 appearances. My work was not without reward, for by 

 increasing the angle of my axial illuminating cone, I found that 

 the black round dot appearance gave place to a reddish square 

 hole in the silex. The dot that was missing showed that that 

 hole had been filled up or coated over with silex. Afterwards 

 I saw a spicule of silex sticking into one of the perforations. 

 Latterly I have discovered a very minute bar of silex stretch- 

 ing across one of the perforations, dividing it into two nearly 

 equal portions. This constitutes probably the smallest, as well 

 as the most difficult, point of detail I have ever seen with the 

 microscope. The apertures count 24,000 per inch in one direc- 

 tion, and 29,100 in the other direction. To find the size of a 

 single aperture is a more difficult matter. If you measure 

 it by the wire micrometer the thickness of the wire renders 

 it difficult to see the edge of the hole. By this means, how- 

 ever, I got a measurement of -553 015- i ncn - I estimate that the 

 diameter of the hole is about equal to the thickness of the inter- 

 vening silex. This would give ^-g^o inch as the diameter. 



The thickness of the bar has been estimated at ~ of the 

 diameter of the hole. Taking the largest measurement of this, 

 viz., ^53o~o inch, would give -^tVo^o inch as the thickness of the 

 bar. I have lately very carefully re-examined this object with 

 the view of estimating its size, and I feel confident that this 

 measurement, if it errs at all, is, if anything, too large. 



I have previously given it as my opinion that the P. 

 formosum was composed of a square grating, but since my 

 discovery of the bar I have modified those views. I now know 

 that I was taking too deep a focus, and I am of opinion that the 

 perforations are circular, or nearly so, on the exterior surface of 

 the valve, and that they cone off to a square grating on the 

 under surface ; in other words they are funnel-shaped, with the 

 small end of the funnel circular and towards the exterior of 

 the valve, the large and square-shaped end towards the interior. 



Some will say that all this is a work of supererogation, the 



