T. F. SMITH ON THE VALVE OP PLEUROSIGMA. 303 



bars at right angles to the length and breadth of the valve. 

 One — I cannot tell whether on the inner or outer side — is very 

 delicate, and seems simply a support for a series of small rings, 

 set diagonally on it ; which rings, being placed immediately over 

 another set of beads or perforations, impart a reddish tinge to 

 that side of the valve, and a general appearance of indefinite- 

 ness. The grating on the other side is more robust, and seems 

 to be a plate of silex, with square perforations, frequently filled 

 at the corners with a secondary deposit, thus forming a round 

 hole. I do not claim to have discovered this grating, as Mr. 

 Nelson mentions it repeatedly, but what I do claim to have dis- 

 covered, is that each alternate hole throws an image in a 

 different focal plane. With the apochromatic T V n -> the one set 

 are of a pale red, on the same plane as the grating, and are, I 

 believe, simply the perforations themselves in the middle of 

 each alternate square. The other series of beads are blue, and 

 thrown inwards from each alternate hole in the grating. It is 

 only when the valve is acted upon in some way — by moisture, I 

 presume — that the grating is visible, and in that state, while 

 the red ones are perfect, the blue seem mostly obliterated, but 

 enough remain to determine the relative positions of the two 

 sets. "When, however, that side of the valve is perfect, the 

 grating disappears, and its place is taken up with red perfora- 

 tions inwards and white interspaces, which, on focussing inwards, 

 turn into blue beads, giving thus two complete sets of secondary 

 structure, similar in shape, but different in colour, and in the 

 position they occupy. 



As far as the outer layers are concerned, I am sure of my 

 ground. I am also sure of this middle plane of blue "beads," 

 and, I think, also of another diagonal grating ; but I do not know 

 if this is the whole of the structure. You are in this position 

 with an oil-immersion objective, on an object mounted dry. If 

 the slightest layer of air intervenes between the different parts 

 of the structure you cannot focus through ; and if it is mounted 

 in balsam the finer parts are obliterated altogether. Neither 

 do I know if the middle layer of blue " beads " is made up of 

 focal images only, or is composed of hollow hemispheres of 

 silex, with the base of each resting on the outer, and the 

 rounded tops thrust into each square of the middle,' diagonal 

 grating. Of one thing I am sure. It is this middle layer of 



