297 



rangement enables me to throw the beam on the object at all 

 angles of incidence, whether from beneath, as in the case of 

 translucent, or from above, in the case of opaque objects, and 

 as the sector is graduated, I have the power of observing and 

 restoring any position at pleasure. 



" To fulfil the second condition, the stage of the micro- 

 scope is made to revolve round the optical axis, and in a plane 

 perpendicular to it. This is effected by constructing the stage 

 on entirely a new plan, in which the slow motions are obtained 

 from concentric rings forming a part of the stage itself, and 

 equally available in every position of the latter. By this ar- 

 rangement the beam of light may be thrown on an object in 

 any azimuth, and a suitable graduation of the stage enables 

 the observer to register and restore its position at any time. 



" Hereafter," continued Mr. Grubb, " I hope to lay be- 

 fore the Academy some of the results obtained by the use of 

 these arrangements ; at present I shall conclude by observing 

 that they are of a highly interesting character, and likely to 

 lead to important discoveries." 



Dr. Allman presented to the Academy the results of some 

 unfinished observations he is at present engaged in on the 

 Claviform and Sertularian Zoophytes. He has found that the 

 medusoid structure, hitherto supposed to be confined to the free 

 locomotive gemmae of these animals, exists also in the fixed 

 ovisacs, though generally so far disguised as to render it easily 

 overlooked. This structure he has found in Coryne, Synco- 

 ryne,Tubularia, Cordylophora, and Sertularia; and he believed 

 himself justified in generalizing the observed facts into the pro- 

 position that a medusoid structure in some form is necessary 

 in these zoophytes for the production of true ova. 



