162 MANAGEMENT OF THE MICROSCOPE. 



equally well adapted to lamp-light, when used in combination with 

 the Bull's-eye (§ 91). Direct sunlight cannot be employed without 

 the production of an injurious glare, and the risk of burning the 

 object ; but the sunlight reflected from a bright cloud is the best 

 light possible. When a Condensing Lens is used, it should always 

 be placed at right angles to the direction of the illuminating rays, 

 and at a distance from the object which will be determined by the 

 size of the surface to be illuminated and by the kind of light 

 required. If the magnifying power employed be high, and the 

 field of view be consequently limited, it will be desirable so to 

 adjust the lens as to bring the cone of rays to a point upon the part 

 of the object under examination ; and this adjustment can only be 

 rightly made whilst the object is kept in view under the Microscope, 

 the Condenser being moved in various modes until that position has 

 been found for it in which it gives the best light. If, on the other 

 hand, the power be low, and it be desired to spread the light equably 

 over a large field, the Condenser should be placed either within or 

 beyond its focal distance ; and here, too, the best position will be 

 ascertained by trial. It will often be desirable also to vary both 

 the obliquity of the light and the direction in which it falls upon 

 the object ; the aspect of which is greatly affected by the manner 

 in which the shadows are projected upon its surface, and in which 

 the lights are reflected from the various points of it. Many objects, 

 indeed, which are distinguished by their striking appearance when 

 the light falls upon them on one side, are entirely destitute both of 

 brilliancy of colour and of sharpness of outline when illuminated 

 from the opposite side. Hence it is always desirable to try the 

 effect of changing the position of the object ; which, if it be 

 ' mounted,' may be first shifted by merely reversing the place of 

 the two ends of the slide, and then, if this be not satisfactory, 

 may be more completely as well as more gradually altered, by 

 making the object-platform itself to revolve. With regard to the 

 obliquity of the illuminating rays, it is well to remark, that if the 

 object be ' mounted ' under a glass cover, and the incident rays fall 

 at too great an angle with the perpendicular, a large proportion of 

 them will be reflected, and the brilliancy of the object will be greatly 

 impaired ; and hence when Opaque objects are being examined 

 under high powers, which can only be done with a very oblique 

 illuminating pencil, they should always be uncovered. 



123. The same general arrangement must be made when Arti- 

 ficial light is used for the illumination of Opaque objects ; the 

 Lamp being placed in such a position in regard to the Stage that 

 its rays may fall in the direction indicated in Fig. 89, and these 

 rays being collected and concentrated by the Condenser, as already 

 directed. Since the rays proceeding from a lamp within a short 

 distance are already diverging, they will not be brought by the 

 Condenser to such speedy convergence as are the parallel rays of 

 daylight; and it must, therefore, be further removed from the 



