172 MANAGEMENT OF THE MICROSCOPE. 



ascertaining, whether the view which he may obtain of them, is 

 in any degree thus marred by incident light. 



88. Although the illumination afforded by the mirror alone is 

 quite adequate for a very large proportion of the purposes for 

 which the Microscope may be profitably employed (nothing else 

 having been used by many of those who have made most valua- 

 ble 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 advan- 

 tage is gained from the use of the Achromatic Condenser. The 

 various modes in which this may be constructed, and may be 

 fitted to the Microscope, have been already described ( 56) ; we 

 have now % to speak of the manner of using it. The lenses with 

 which the Condenser is provided should be made to separate 

 from each other, in such a manner that two or three distinct 

 powers should be afforded ; the complete combination should be 

 used with objectives of l-5th inch focus or less ; the front lens 

 should be removed with objectives of from half to a quarter of 

 an inch focus ; and the second lens may be removed, so that the 

 back lens will be alone employed, when it is desired to use the 

 condenser with objectives of less than half an inch focus. It is 

 of the greatest importance that the Condenser should be ac- 

 curately adjusted, both as to the coincidence of its optical axis 

 with that of the Microscope itself, and as to its focal distance 

 from the object. The centring may be most readily accom- 

 plished, 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 Microscope with an ob- 

 jective of sufficiently low power to take in the whole of it, if 

 this circle be not found to be 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. The focal adjust- 

 ment, 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 window-bar, if daylight be em- 

 ployed, or of the top, bottom, or edge of the lamp-flame, if lamp- 

 light be in use ; such a vertical movement should be given to the 

 condenser, as may render the view of this as distinct as possible ; 

 and the direction of the mirror should then be sufficiently 

 changed to displace these images, and to substitute for them the 

 clearest light that can be obtained. It will generally be found, 

 however, that although such an exact focussing gives the most 

 perfect results by daylight, yet that by lamp-light the best illu- 

 mination is obtained, when the condenser is removed to a some- 

 what greater distance from the object, than that at which it gives 



