MANAGEMENT OF THE MICROSCOPE. . 145 



(blackened on the side towards the stage) between the window and the 

 object; care being of course taken that the screen does not interfere with 

 the passage of light to the mirror. Such a screen may be easily shaped 

 .and adapted either to be carried by the stage itself, or by the stand for 

 the condenser; but it is seldom employed by Microscopists, as it inter- 

 feres with access to the left side of the stage; and the interposition of 

 the hand, so often as it may be needed, is more frequently had recourse 

 to in preference, as the more convenient expedient. The young Micro- 

 scopist who may be examining transparent objects by daylight, is recom- 

 mended never to omit ascertaining whether the view which he may 

 obtain of them is in any degree thus marred by incident light. 



144. Although the illumination afforded by the Mirror alone is quite 

 adequate for a very large proportion of the purposes for which the Micro- 

 scope may be profitably employed (nothing else having been used by 

 many of those who have made most valuable contributions to Science by 

 means of this instrument), yet, when high magnifying powers are 

 employed, and sometimes even when but a very moderate amplification 

 is needed, great advantage is gained from the use of a Condenser. The 

 form which has been described under the name of the Webster Condenser 

 ( 100) answers so well for most purposes, and may in addition be so 

 easily converted into a ' black ground' Illuminator, that the working 

 Microscopist will find it convenient to keep it always in place; substitut- 

 ing an Achromatic Condenser of greater power ( 99) only when specially 

 needed. Special care is needed in the use of this last, both as to the 

 coincidence of its optic axis with that of the Microscope itself, and as to 

 its focal distance from the object. The centering may be most readily 

 .accomplished by so adjusting the distance of the Condenser from the 

 Stage (by the rack-and-pinion action or the sliding movement with which 

 it is always provided), that a sharp circle of light shall be thrown on any 

 .semi-transparent medium laid upon it; then, on this being viewed 

 through the Microsope with an Objective of sufficiently low power to take 

 in the whole of it, if this circle be not found concentric with the field of 

 view, the axis of the Condenser must be altered by means of the milled- 

 head tangent-screws with which it is provided. Or a cap with a minute 

 central aperture may be fitted on the top of the Condenser, and this 

 aperture centered in the field of an objective of medium power. Or, 

 again, a diaphragm with a very minute central perforation may be placed 

 at a little distance beneath the Achromatic Condenser, and the image of 

 this may be brought into the centre of the field of a l-4th objective, 

 which is the best arrangement when it is to be used with very high powers. 

 The focal adjustment of the Condenser, on the other hand, must be made 

 under the Objective which is to be employed in the examination of the 

 object, by turning the Mirror in such a manner as to throw upon the 

 visual image of the object (previously brought into the focus of the Micro- 

 scope) an image of a chimney or a window-bar, if daylight be employed, 

 or of the top, bottom, or edge of the lamp-flame, if lamp-light be in use; 

 the focus of the condenser should then be so adjusted as to render the 

 view of this as distinct as possible; and the direction of the Mirror should 

 then be sufficiently changed to displace the image, and to substitute for 

 it the clearest light that can be obtained. It will generally be found, how- 

 ever, that although such an exact focussing gives the most perfect results 

 by Daylight, yet that by Lamp-light the best illumination is obtained 

 when the Condenser is removed to a somewhat greater distance from the 

 object, than that at which it gives a distinct image of the lamp. In every 

 10 



