189I.] NEW-YORK MICROSCOPICAL SOCIETY. 71 



layers; but where I have taken an under layer on the same valve, 

 air comes between and the image has suffered — will, in fact, be 

 recognized by you at once by its dull tone, and I wish you to draw 

 no other inference from it but to see that there is another struc- 

 ture underneath. 



On the slide No. 34 will be found in the centre a single layer 

 of a fine species allied to P. angulatu>n, and at the side a part, I be- 

 lieve, of the same valve with the both layers intact ; and a study 

 of the changes which take place in both on focussing up and 

 down enables me to draw certain deductions, for my own guid- 

 ance at least. 



In the single layer there are no changes whatever beyond a 

 change from a positive to a negative image up and down, and 

 then the object vanishes altogether ; but on the other fragment 

 there are six changes at least, proving the structure to be more 

 complicated. 



It will then, I think, be safe to argue that when the object 

 under investigation changes from more than a positive to a nega- 

 tive image ; when, after losing, or almost losing it, you come upon 

 another distinct image, either higher or lower, it proves this, at 

 least : that the structure you are examining is a compound one, 

 although I need not argue from it that the appearance you get is 

 necessarily the true one. 



The inference I would draw, then, from certain appearances 

 on the surface, would be a limited one, but within those limits to 

 be trusted. Thus, where you see squares or hexagons, it is not 

 necessary that there should be squares or hexagons in the struc- 

 ture ; but it may be safely inferred that the recurring distances 

 of the image represent the true distances of the actual structure. 

 There is, in fact, with any legitimate use of the objective, no 

 doubling and trebling of the lines as represented in influential 

 quarters, but a correct rendering of distances if within the resolv- 

 ing power of the aperture, as shown by me in my " Note on the 

 Abbe Diffraction Plate," published in the July number of the 

 Quekett Joiirnaliox 1889. 



Knowing my Abbe Diffraction Plate well, then, I have come to 

 no conclusions on appearances only, and have formed no definite 

 opinions on any structure until I have seen it isolated ; and can 

 leave the results with all the more confidence in your hands. 



