170 COMPARATIVE VALUE OF OBJECT-GLASSES. 



however, greatly accelerated, and rendered more energetic, by 

 Heat ; and this seems to show that it is due, either directly to 

 some calorical changes continually taking place in the fluid, or to 

 some obscure chemical action between the solid particles and the 

 fluid which is indirectly promoted by heat. It is curious that 

 the closer the conformity between the specific gravity of the solid 

 particles and that of the liquid, the less minute need be that 

 reduction in their size which is a necessary condition of their 

 movement ; and it is from this that the substances just named are 

 so favourable for the exhibition of it. On the other hand, the 

 particles of Metals, which are from seven to twelve times as 

 heavy as water, require to be reduced to a minuteness many 

 times greater than that of the particles of carmine or gamboge, 

 before they become subject to this curious action. In any case in 

 which the motions of very minute particles, of whatever kind, are 

 in question, it is necessary to make allowance for this 'molecular 

 movement; ' and the young Microscopist will therefore do well to 

 familiarize himself with its ordinary characters, by the careful 

 observation of it in such cases as those just named, and in any 

 others in which he may meet with it. 



131. Comparative Values of Object-Glasses; Test- Objects. — 

 In estimating the comparative values of different Object-glasses, 

 regard must always be had to the purpose for which each is de- 

 signed ; since it is impossible to construct a combination which 

 shall be equally serviceable for every requirement. It is commonly 

 assumed that an Objective which will show certain Test-objects 

 must be very superior for everything else to a glass which will 

 not show these ; but this is known to every practical Microscopist 

 to be a great mistake, — the qualities which enable it to resolve 

 the more difficult ' tests ' not being by any means identical with 

 those which make it most useful in all the ordinary purposes of 

 Scientific investigation. Four distinct attributes have to be spe- 

 cially considered in judging of the character of an Object-glass, 

 viz. — (1) its defining power, or power of giving a clear and distinct 

 image of all well-marked features of an object, especially of its 

 boundaries ; (2) its penetrating power, or focal depth, by which 

 the observer is enabled to look into the structure of objects ; (3) 

 its resolving poiver, by which it enables closely-approximated 

 markings to be distinguished ; and (4) the flatness of the field 

 which it gives. 



i. The ' Defining power ' of an Objective mainly depends upon 

 the completeness of its corrections, both for Spherical and for 

 Chromatic aberration (§§ 9-15) ; and it is an attribute essential 

 to the satisfactory performance of any Objective, whatever be its 

 other qualities. Good definition may be more easily obtained with 

 lenses of small or moderate than with lenses of large angular 

 aperture ; and in the aim to extend the aperture, the perfection 

 of the definition is not unfrequently impaired. An experienced 



