189I..] NEW-YORK MICROSCOPICAL SOCIETY. 69 



pies of torn structure from that side of the valve, beyond a 

 notched edge, 1 do not care to speak absolutely. Other exam- 

 ples, however, from the outer side, will show you how hexagons 

 can be made, and you can form your conclusions accordingly. 



Slides Nos. 40, 41, and 42 are taken at different focal planes on 

 the same valve, which shows the surface abraded and two isolated 

 fibrils running lengthways on the valve. The difference of ap- 

 pearance on the sound parts is considerable and varies from long 

 rectangles on the upper focus to decided hexagons on the lower, 

 while the only difference it makes on the fibrils themselves is just 

 to thicken them a little. The same thing happens also on slides 

 Nos. 47 and 48, taken with a little difference of focus, where 

 we have squares on the upper and hexagons on the lower, while 

 the free ends of the torn fibrils suffer but little change. 



All my remarks up to now apply to the species having diagonal 

 "markings" only; but the last slide of the list is taken from 

 F. baliicum, as an attempt to give a general idea of the difference 

 of the structure between the two. Here we have still fibrils, but 

 apparently of one continuous strand with swellings at regular in- 

 tervals, which, lying side by side of the others, with the knobs 

 nearly touching, give the appearance of squares. 



In conclusion it may be necessary for me to say something 

 about my methods of investigation, that no outstanding doubts may 

 be left as to the correctness of my conclusions. This is all the 

 more binding on me, as Dr. Cox has kindly told me that " I had 

 need to tell the details of my examination so fu ly as to repel the 

 possibility of doubt that the matter is in no sense illusory." I ac- 

 cept the invitation most cordially, as every practical microscopist 

 will know that the method of investigation is everything. 



First, as to illumination : I use a strictly central cone of light, 

 collected first from the edge of the lamp flame by a bull's-eye 

 condenser before being further condensed by the achromatic con- 

 denser. In this way I get a more intense illumination. Indeed, 

 I found it impossible to differentiate some of the structure with- 

 out it. My achromatic condenser is a dry one, and with the 

 bull's-eye will give me all the light the lens will stand. In 

 theory, of course, the whole of the aperture can only be utilized 

 by the aid of an oil-immersion condenser. Granted the back 

 lens of the combination can be only filled that way ; but the 



