THE OPTICAL EQUIPMENT 65 



obscures one-half of the front lens of the object-glass, but by 

 throwing out the auxiliary reflector it admits of a rapid change 

 of illumination from oblique to vertical light ; it is therefore 

 very useful for ordinary purposes of observation. 



Another arrangement for top-lighting which, however, is 

 only applicable to small objects is that known as the Lieber- 

 kuhn. This is a concave silver reflector which slides on to the 

 front of the object-glass ; this again must be made for the 

 particular objective in use, as its curve must be varied to suit 

 the focal length of the latter or more exactly to suit its working 

 distance. The illumination is effected from below the stage, 

 the light being projected upwards by the flat surface of the 

 mirror on the microscope as for transparent objects. The 

 object must therefore be so small that it allows a sufficiency 

 of light to pass round it for the Lieberkuhn to be completely 

 filled. The light on striking the silvered surface is reflected 

 from all sides and condensed on to the object, and the best 

 results are obtained when a dark stop is placed below the 

 object, intercepting the central portion of the incident beam 

 which is not required. The performance of this apparatus is 

 only at its best when all the conditions indicated are fulfilled ; 

 but it then forms for low-power work a very useful method of 

 illuminating opaque objects in cases where such illumination 

 is required to be incident from all sides. It is obviously only 

 suitable to low or medium powers. 



The simplest and most convenient method of obtaining 

 normal illumination with low powers is by interposing between 

 the object and the objective a cover-glass, or piece of thin 

 optically worked glass, which is placed at an angle of 45 to the 

 optical axis (Fig. 17). The light is thrown in the direction as 

 shown, striking the reflecting glass surface K, and is partially 

 thrown down on to the object M. It is then reflected from M 

 in the direction of the optical axis, and is received by the 

 objective 0. The appliance is clamped on to the front 

 of the object-glass by a metal ring H, and it is only 

 necessary to ensure that the mount of the objective is of 

 such a size that it will allow the reflector to slip over it. 

 This small appliance is perfectly efficient for any power 

 where there is sufficient working distance for it to be placed 

 in position. The illuminant with which it is used must be 



