290 W. P. DOLLMAX OX STEREO-PHOTO-MICROGRAPHS. 



the underside of the cover-glass of the object and out of focus. 

 It was noticed that on the right-hand half of the print this 

 object took this shape ( and on the left is curved in the opposite 

 direction, thus ). The true image should have been a straight, 

 upright line. This distortion was due, no doubt, to the tendency 

 of out-of -focus images to assume the form of the aperture through 

 which the rays pass (in this case semicircular), thus bending the 

 out-of-focus image of the straight line in opposite directions, when 

 this image was formed by the opposite halves of a circular lens. 



In this description I fear I have been rather diffuse, and have 

 detailed much that is perhaps generally known ; but I thought 

 it expedient, owing to my inability to be present at the meeting 

 when this paper will be read, to be a little particular. 



EXPLAXATION OF PLATE 21, 



Polyzoa — Idmonea radians, x 9. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. IX., Ao. 18, April 190C. 



