ANGULAR APERTURE. 



ANIMAL KINGDOM. 



This question will be best answered by ex- 

 periment. If we take a fragment of the 

 valve of an Isthmia, and examine it under a 

 high power of small aperture, all the parts 

 are very distinctly seen by the ordinary light 

 of the mirror ; and the various depths of 

 shadow of the different parts of the depres- 

 sions and the undepressed portions render 

 these also clearly distinguishable (PL 13. 

 fig.2c); and when an object-glass of very large 

 aperture is used, the distinctness is rather 

 impaired than improved. But if we examine 

 a fragment of the valve of a Gyrosigma, and 

 this requires an object-glass of large aperture 

 to render the markings visible, no distinction 

 of the various parts of the depressions and 

 the undepressed portions is visible ; all we 

 see is, that the depressions, as a whole, are 

 dark and the undepressed portions are lumi- 

 nous (PL 11. figs. 39, 40, 48). Hence the 

 Isthmia requires defining power, whilst the 

 Gyrosiyma requires penetrating power and 

 large angle of aperture, to exhibit the mark- 

 ings ; yet the structures differ only in size. 

 And there can be no doubt that if we could 

 examine the valve of the Gyrosigma under a 

 power as high relatively to the size of the 

 depressions, as that under which we can 

 examine the Isthmia, the same relations 

 being preserved between the angle of aper- 

 ture of the object-glass and the angular in- 

 clination of the refracted rays, the various 

 parts of the depressions and the undepressed 

 portions would be equally recognizable in 

 both cases. 



The same relation applies to fine lines 

 scratched or etched upon glass. It was 

 noticed by Dr. Goring, that although the 

 lines on the scales of insects required an 

 object-glass of comparatively large aperture 

 to show them, yet those existing upon glass 

 micrometers did not so. But this statement 

 is only partially correct; for although the 

 coarser lines upon micrometers are well seen 

 under 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. 



Hence it is evident that large angular 

 aperture or penetrating power in an object- 

 glass is a substitute, and a very imperfect 

 one, for defining power. This most import- 

 ant point has not hitherto been noticed; and 

 it is to be hoped that our object-glass 

 makers will direct their earnest attention to 

 it, and endeavour to supply those who are 

 engaged in microscopic researches with that, 



to the production of a very imperfect substi- 

 tute for which, all their energy appears at 

 present devoted. 



In conclusion, it may be noticed that these 

 remarks have been principally confined to one 

 class of objects requiring penetrating power, 

 viz. the valves of the Diatomacea3. This has 

 been done advisedly, because the scales of 

 Insects, which may be regarded as forming 

 the type of the other class, involve consi- 

 derations of a mixed kind, which would have 

 tended to confuse the subject. The longi- 

 tudinal ridges upon the scales of Insects, in 

 regard to their relation to penetration, may 

 be viewed in the same light as the unde- 

 pressed portions of the valves of the Diato- 

 macese; and the same explanation will apply 

 to the visibility of the one as to the other, 

 under the various conditions. 



The transverse lines seen upon the scales 

 of insects, will be noticed under SCALES OF 

 INSECTS. The structure of the valves of 

 the DIATOMACE^E is discussed under that 

 head; see also INTRODUCTION, p. 33, /. 



We have thought it better to refer the 

 angular apertures of the various object- 

 glasses to the article OBJECT-GLASSES. 



The above article consists principally of 

 an abstract from a paper read before the 

 Royal Society of London on the 1 1th May, 



BIBL. Wenham, Trans. Micr. Soc. iii. 

 p. 83, April 1850 and April 1854; Rainey, 

 ibid., Oct. 1853 and Jan. 1854; Gillett, 

 Proceedings of Royal Society, vii. 16. 

 March 1854; Lister, Phil. Trans, cxxi.; 

 Goring, Micrographia ; Select Works ; Jour- 

 nal of the Royal Institution, xxii. ; Pritchard, 

 Microscopic Cabinet ; Griffith, Proceedings 

 of Royal Society, vii. 60. May 1854. 



ANIMAL. The definition of an animal 

 in reference to the distinction from vege- 

 tables is discussed in the article VEGE- 

 TABLES. 



ANIMALCULE. A little animal ; a 

 term usually applied to the species of Infu- 

 soria, Rotatoria, &c. It was formerly applied 

 also to many of the lower Algae. The Latin 

 term animalculum (with the plural animal- 

 cula) is frequently met with. 



ANIMAL KINGDOM. In accordance 

 with our plan, as laid down in the Preface, 

 we give here a tabular view of the animal 

 kingdom, so that the position of the various 

 classes and orders alluded to in various parts 

 of this work, may be readily found. Those 

 classes, orders, families and genera to which 

 particular interest is attached, in relation to 



