1885.] NEW-YORK MICROSCOPICAL SOCIETY. 37 



the mirror. In like manner, images will be made by other trans- 

 parent bodies, or by the transparent parts of any structure, which 

 are of lenticular or globular form. 



Concave lenses, as well as convex, will give images, but with 

 this difference, — the images will be found below the plane of 

 the foci of the lenses, and will be inverted ; whereas, images pro- 

 duced by convex lenses are formed erect and, as before stated, 

 at a plane above such focus. It follows that air-bubbles in water 

 will yield inverted images, the water immediately surrounding 

 them acting as a biconcave lens. 



These facts may possibly be of some service in determining 

 the character of minute bodies or structures, such, for example, 

 as human blood corpuscles, all of which show erect images — a 

 proof that they are nucleated or, at least, lenticular at the centre. 

 The head of the pin-shaped sponge-spicule, and the nuclei in 

 certain diatoms, produce inverted images. 



HETEROMEYENIA RYDERI. 



BY PROF. SAMUEL LOCKWOOD, PH.D. 

 {Read January ibth, 1885.) 



While summering in 1883 at Twin Mountain, N. H., Mr. F. 

 W. Devoe and myself did some object-hunting for the micro- 

 scope. On pulling up some submerged sticks from a still pond 

 in a field, we observed that they were encrusted with certain 

 green, moss-like prominences, which proved to be specimens of 

 a fresh-water sponge. The species has been determined by Mr. 

 Edward Potts to be Heteromeyenia Ryderi. Mr. Devoe has pre- 

 pared an interesting mount of the sponge, on which, at this 

 gentleman's request, a few remarks are herewith offered. 



By reason of the green color, the amateur, on first seeing a 

 specimen of Spongilla, is invariably deluded into the belief that 

 he has found a peculiar species of confervoid alga. The color 

 is owing to the abundance of chlorophyll in the sarcode of the 

 sponge. This sarcode, or pseudo-flesh, is composed largely of 

 undifferentiated, that is, structureless, protoplasm ; the rest, con- 

 sisting of the differentiated protoplasm, is composed of flagellate 

 cells not unlike ciliated monads. 



It may be remarked that the Spongida, or Porifera, are divided 



