375 



1 1 th. Reproduction, gemmules, &c. Under this head the ovaries, 

 ova, and gemmular modes of propagation, are described under five 

 heads : 



1st. Ova without an ovarium. 



2nd. Ova generated within ovaria. 



3rd. Gemmules secreted within the sponge. 



4th. Gemmules produced externally. 



5th. By spontaneous division of the sarcode. 



The remaining portions of the paper, consisting of observations on 

 the generic characters, the specific characters, and on the mode 

 of examination, will form the subject of a future communication. 



XVII. " Further Observations upon the Nebulae, with practical 

 details relating to the Construction of Large Telescopes." 

 By the EABL OF ROSSE, F.R.S. Received June 5, 1861. 

 (Abstract.) 



In this paper the processes which were found best to answer in 

 casting specula of 6-feet aperture have been described at some length ; 

 and the precautions which were taken to prevent accidents during 

 the progressive stages of manufacture. 



Some information, which may perhaps be useful, has also been 

 gathered from the memoranda kept on each occasion when the spe- 

 cula were ground and polished ; and the results of a long experience 

 in the different manipulations have been put together in a practical 

 form, so as best to enable the civil engineer to undertake the con- 

 struction of large telescopes as a matter of business, 



A few hints have been given on figuring and repolishing large sur- 

 faces, which the astronomer may find useful should his services be 

 required in distant countries. 



An attempt has been made to define the extreme limits to which 

 telescopic vision may be pushed in this country, in answer to the 

 question whether instruments of greater dimensions might not be 

 advantageously constructed. 



The peculiarities in the mounting of the large telescope are slightly 

 noticed, and their advantages and disadvantages explained, as they 

 have been brought out by experience. 



This is followed by a selection from the observations made during 



