Accessories. 55 



of the diaphragm of the condenser until one quarter of the 

 back of the objective is shut out, again put in the eyepiece, 

 and this will be found to give the most accurate results. 

 Of course, this may be varied according to the transparency 

 or opaqueness of the object under view. 



Condensers having a numerical aperture of 1*0 and over 

 require to be immersed in order to get their full aperture ; 

 that is, a drop of immersion oil or Canada balsam should 

 be placed between the top lens of the condenser and the 

 object. It will be found generally that the condenser is a 

 little too long in focus for this to be effected. Under such 

 circumstances an additional 3x1 slip should be placed 

 under the object, when the oil contact will be easily main- 

 tained, but the distance will vary according to the thick- 

 ness of the slip on which the object is mounted, and the 

 intermediate contact slip would have to be selected ac- 

 cordingly. With the majority of condensers there are 

 also fitted diaphragms having the centre blocked out, with 

 which dark-ground illumination is obtained. This gives a 

 beautiful effect to very transparent objects, such as infusoria, 

 pond life specimens, etc. In the Abbe illuminator a cell 

 for this is generally provided just over the iris diaphragm 

 and beneath the condenser lenses. One of these dark- 

 ground stops should be fitted into the ring, the iris 

 diaphragm opened completely, the condenser having been 

 previously adjusted in the usual way, when it will be 

 found that the object will be illuminated ; but the ground 

 on which it is seen will be black. Different objectives 

 require different sizes of diaphragms. For this purpose 

 blacked cardboard may be easily cut to the size and adapted 

 exactly to suit the objective. For oblique illumination one 

 of the stops having a crescent or a slight slot is fitted in 

 this ring, the iris diaphragm being open completely, when 

 it will be found that striated objects will be exhibited in 

 a very concise manner. That the employment of a con- 

 denser is an absolute necessity cannot be too much im- 

 pressed. No good results can be obtained without it. 



