J, E. INGPEN ON A '' FALSE-LIGHT EXCLUDER." 263 



and the resolving power in no way injured. When the cap was 

 slipped up close to the olijective the angle was reduced from 100^ 

 to 70°, witli far finer definition than when limited to the same angle 

 by a stop at the back. 80 completely is the stray light stopped 

 out that in many cases the diaphragm may be dispensed with. 



The nearest approach to the kind of definition above mentioned 

 was obtained by the Continental method of using a very small 

 aperture as close under the object as possible, instead of a larger 

 one lower down, as is usual in this coimtry ; and this method might 

 often be used with advantage where Br cap could not be placed over 

 the objective. This method, in its best form, was first proposed 

 by Mr. Yarley many years ago, and called by him a " dark 

 " chamber." Though long since forgotten and disused, it might be 

 well worthy of re-introduction now that quality of illumination is 

 considered to be a matter of primary importance. 



On the Development of Hydra Vnlgaris. 



By James Fullagae, Esq., Assistant Secretary, East Kent 

 Natural History Society, Canterbury. 



Communicated by Mr. Curties. 



Read April 2Uh, 1874. 



It appears from the statements of various writers on Natural 

 History, that the development of Hydra from ova has never, or 

 very rarely been witnessed, though, at the same time, no doubt is 

 entertained of the fact that one of the means of its reproduction is 

 from ova. I have had H. vulgaris and H. viridis^ under my 

 observation for more than three years past, and, after various 

 failures and disappointments, I have at length succeeded in wit- 

 nessing the hatching out of several specimens of Hydra vulgaris, 

 an account of which will, I hope, be interesting to the Quekett 

 Microscopical Club. 



Hydra vulgaris differs in many respects from Hydra viridis. 

 The Qgg is larger, and studded with what appear to be short spines 

 (pi. xii., fig. 1, h). The shell is not smooth, nor is it covered witli 

 an irregular network, as in H. viridis, but it is surrounded witli a 

 transparent gelatinous envelope, which it retains to the time of 



* A paper on the Development of Hydra Viridis from Ova will be found in 

 " Science Gossip " for 1873, pp. 12 and 175. 



