THE OPTICAL EQUIPMENT 67 



itself on to the object, a critical image of the illuminant 

 being formed at the surface of the object. The objective thus 

 performs a double function ; it acts in effect as a condenser, 

 while also serving its ordinary purpose of image-formation 

 and magnification of the image. The surface of the specimen 

 may then be viewed through the eye-piece of the microscope 

 in the ordinary way. Whether the cover-glass or the prism is 

 the better form to use is a controversial question ; but the prism 

 arrangement certainly has the objection that one-half only of 

 the objective is in use for the purpose of image-formation. The 

 prism is so arranged that it projects over one-half of the 

 back lens of objective, and this half therefore serves as the 

 condenser ; the remaining half forms the ordinary microscopic 

 image. In general, it will be found that the arrangement in 

 which a cover-glass is used has some slight advantage, as 

 there is no interference with the aperture of the objective. 

 The carrier containing the reflecting arrangement is made so 

 that it may be rotated, and the angle of incidence of the light 

 may be varied, within limits, at will. The radiant should be 

 approximately eight to ten inches from the microscope, since 

 it is only at a short distance such as this that the objective 

 will efficiently condense the light on to the object. To obtain 

 exactly vertical illumination the illuminator should be in such 

 a position that its reflecting-surface is at an angle of 45 to the 

 vertical. 



A considerably improved vertical illuminator has been 

 recently brought out by Messrs. E. Leitz (Fig. 19). It consists 

 essentially of the same arrangement as has already been 

 described, viz. a prism vertical illuminator. It has, however, 

 inserted at one side and in the direction in which the light is 

 thrown on to the prism, a subsidiary tube carrying a small 

 condensing-lens L, which is adjustable in position in the direc- 

 tion of the length of the" tube. To be used in conjunction with 

 this appliance is a separate arrangement consisting of two 

 mirrors ; the lower one S 1 is adjustable in any direction, the 

 upper one S 2 is arranged at an angle of 45 to the vertical. 

 The incident light, therefore, on striking the first mirror is 

 reflected on to the second one and thrown into the condensing 

 lens L of the vertical illuminator. The advantage of the 

 arrangement is that the position of the source of light in 



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