88 PHOTO-MICROGRAPHY 



out to the reader how he can learn to ascertain this for himself in any given lens 

 before he purchases it. 



There are two methods for dry objectives, but only one for homogeneous ones. 



The first, applying to both types of lenses, is by using a special apparatus devised 

 for the purpose by Professor Abbe, called the apertometer (Fig. 47). This consists 

 of a piece of thick glass about 3 inches in diameter, half an inch thick. The part 

 where the glass becomes segmental is bevelled from above downwards to an angle of 

 about 45°. Near the centre (marked a) is a small disc of silvered glass with a tiny 



Fig. 47 



hole in its centre, where the silvering is removed. Two plates of metal, which can be 

 shifted around the outer edge of the glass shown at b b are square one side and 

 pointed on the other (see b in the side view). To use this apparatus, the glass is 

 placed with the graduated surface uppermost upon the stage of the microscope (fixed 

 vertically) in such a manner that the circular portion is forwards and the chord or 

 bevelled piece backwards, towards the stem of the instrument. The edges of the 

 little hole are now focussed with the objective to be measured, an eye-piece being used, 

 and the length of the draw-tube the same as when the objective is ordinarily in use. 

 The two indices, as b b are called, are then placed on the edge of the glass, as shown 

 in the plate, but close to the middle of the semi-circle. Their sharp points should lie 

 along the vertical edge of the disc, and their flat sides upon its upper graduated 

 surface. It is best to direct the points away from each other to the outer side, if the 

 power to be examined is comparatively high (N. A. above o-6 or 07), but towards 

 each other to the inner side if a low objective is employed. 



With each instrument an auxiliary objective is supplied which screws, or must be 

 made to screw, into the end of the draw-tube, after which both are returned to the 

 microscope, with the auxiliary lens passing down the main tube. The same eye-piece 

 is then placed in the draw tube as before, and the auxiliary microscope then obtained 

 is focussed to the image of the indices by sliding the draw tube in the main tube. 



