176 COMPARATIVE VALUE OF OBJECT-GLASSES. 



formerly considered adequate tests for higher powers, is for ordi- 

 nary purposes rather injurious than beneficial. In estimating the 

 value of an Object-glass, it should always be considered for what 

 purpose it is intended ; and its merits should be judged of according 

 to the degree in which it fulfils that purpose. We shall therefore 

 consider what are the attributes proper to the several ' powers ' of 

 Object-glasses — lotv, medium, and high ; and what are the objects 

 by its mode of exhibiting which it may be fairly judged. 



I. By Object-glasses of low power we may understand any whose 

 focal length is greater than half -an-inch. The 'powers' usually 

 made in this country are known as 3 inch, 2 inch, lj inch, 1 inch, 

 and 2-3rds inch focus ; and they give a range of amplification of 

 from 13 to 60 diameters with the A eye-piece, and of from 

 20 to 90 diameters with the B eye -piece. These are the 

 Objectives which are most used in the examination of Opaque 

 objects, and of Transparent objects of large size and of compara- 

 tively coarse texture ; and the qualities most desirable in them are 

 a sufficiently large Aperture to give a bright image, combined with 

 such accurate Definition as to give a clear image, with Focal Depth 

 sufficient to prevent any moderate inequalities of surface from 

 seriously interfering with the distinctness of the entire picture, and 

 with perfect flatness of the image when the object itself is flat. 

 For the 3 inch, 2 inch, or 1\ inch Objectives,* no ground of judg- 

 ment is better than the manner in which it shows such an injected 

 preparation as the interior of a Frog's Lung (Fig. 387) or a portion 

 of the villous coat of the Monkey's Intestine (Fig. 384) ; for the 

 aperture ought to be sufficient to give a bright image of such 

 objects by ordinary daylight, without the use of any illuminator ; 

 the border of every vessel should be clearly defined, without any 

 thickness or blackness of edge ; every part of such an object that 

 comes within the field should be capable of being made out when 

 the focal adjustment is adapted for any other part ; whilst, by 

 making that adjustment a medium one, the whole should be seen 

 without any marked indistinctness. If the aperture be too small, 

 the image will be dark ; if it be too large, details are brought into 

 view (such as the separateness of the particles of the vermilion 

 injection) which it is of no advantage to see, whilst, through the 

 sacrifice of penetration, those parts of the object which are brought 

 exactly into focus being seen with over-minuteness, the remainder 

 are enveloped in a thick fog through which even their general con- 

 tour can scarcely be seen to loom ; and if the corrections be imper- 

 fectly made, no line or edge will be seen with perfect sharpness. 

 For Defining power, the Author has found the Pollen-grains of the 



* These are ordinarily composed of two pairs of lenses only, as the 

 corrections can be adequately made by this combination for an Angular 

 Aperture of 20°, which is the largest that is found practically useful for 

 the I5 inch. (See p. 161, note.) 



