166 THE MICROSCOPE AND ITS REVELATIONS. 



solve (under deep Eye-pieces) many objects which were formerly consid- 

 ered adequate tests for higher powers, is for ordinary purposes rather 

 injurious than beneficial, detracting from the value of the Objective for 

 the work to which it is specially adapted. For Microscopists of large 

 Biological experience know perfectly well that every i power' has its own 

 proper range and capacity; and that they work most satisfactorily with 

 the ' power ' most suitable to the investigation on which they may be en- 

 gaged. 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 objects proper to the several ' powers * 

 of Object-glasses low, medium, and high; and what are the objects by 

 its mode of exhibiting which, each may be fairly judged. 



I. By Objectives 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 4 inch, 3 inch, 2 inch, 1 inch, 1 inch, and 2 3ds 

 inch focus; and they give a range of amplification of from 10 to 70 dia- 

 meters with the A eye-piece, and of from 16 to 120 diameters with the 

 B eye-piece. An ' adjustable' low power is made by Zeiss of Jena (ob- 

 tainable from Messrs. 'Baker), in which, by varying the position of the 

 front-lens by means of a screw-collar, a range of power is obtainable from 

 about 8 to 16 diameters with the A eye-piece, and from 12 to 24 with the 

 B eye-piece. This has been found by the Author extremely convenient 

 for the display of large opaque objects, of which it is desired to show the 

 whole under as high an amplification as will make their images fill the 

 field. Objectives of low power 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 suffi- 

 ciently large aperture to give a bright image, combined with such accu- 

 rate 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, 1 no ground of judgment is better than the manner in 

 which it shows such an injected preparation as the interior of a Frog's 

 Lung (Fig. 485) or a portion of the villous coat of the Monkey's Intes- 

 tine (Fig. 479); for the aperture ought to be sufficient to give a bright 

 image of such objects by ordinary daylight, without the use of any illu- 

 minator; 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 ad- 

 justment is adapted for any other part; whilst, by making that adjust- 

 ment a medium one, the whole should be seen without any marked in- 

 distinctness. If the Aperture be too small, the image will be dark: but 

 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, tjirough 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 gen- 

 eral contour can scarcely be seen to loom. If the corrections be imper- 



1 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 23, 

 "which is the largest that is found practically useful for the 1 inch. 



