216 Zoological/ Society. 



same relations being preserved between the angle of aperture of the 

 object-glass and the angular inclination of the refracted rays, the 

 various parts of the depressed and undepressed portions would be 

 equally recognizable in both cases. 



This is also true of fine lines scratched or etched on glass ; for 

 although the coarser lines upon glass micrometers are well seen with 

 an object-glass of small aperture with good defining power and 

 direct light, yet the finest lines upon Nobert's test-slide require 

 penetrating power in the object-glass, and oblique light. Large an- 

 gular aperture or penetrating power is but a very imperfect substitute 

 for defining power — an important point which the author believes has 

 not hitherto been noticed, and to which he would invite the earnest 

 attention of object-glass makers. 



The author concludes by observing that his remarks have been 

 principally confined to one class of objects requiring penetrating 

 power, viz. the valves of the Diatomacese. This has been done ad- 

 visedly, because the scales of insects, which may be regarded as form- 

 ing the type of the other class, involve considerations of a mixed 

 kind, which would have tended to confuse the subject. The longi- 

 tudinal ridges upon the scales of insects, in their relation to pene- 

 tration, may be viewed as representing the undepressed portions of 

 the valves of the Diatomacese ; and the same explanation will apply 

 to the visibility of both under various conditions. The transverse 

 lines seen upon the scales are not indications of true structure ; but 

 their origin, as also that of the lines seen upon the valves of the 

 Diatomacese, from circular or angular depressions, does not come 

 within the conditions involved in the principle which it has been the 

 object here to elucidate. It will suffice to say that the true struc- 

 tures producing the appearance of transverse markings upon the 

 scales of insects are best resolved by small angular aperture and'good 

 definition. 



It has been assumed also, that the markings upon the valves of 

 the Diatomacese arise from depressions. This can be proved to be 

 the case in the larger ones (Isthmia, &c); and there is sufficient 

 evidence to render it at least highly probable in the remainder. But 

 this is an unessential point as regards the principle, and therefore it 

 has not been dwelt upon. 



ZOOLOGICAL SOCIETY. 



February 10, 1852.— W. Yarrell, Esq., in the Chair. 



Monograph or the Family Branch ipodid.e, a family 

 of Crustaceans belonging to the Division Entomos- 

 traca, with a description of a new genus and species 

 of the family, and two new species belonging to the 

 Family Limnadiad^e. By W. Baird, M.D., F.L.S. &c. 



Next to the Apodidce, the largest species of Entomostraea belong 

 to the family Branchipodidce. This family contains perhaps the most 



