394 



OBJECTIVES, EVE-PIECES, THE APERTOMETEB 



involves the same principle as that of Tolles. but it is carried out in 

 a simpler manner. 



Abbe's instrument is presented in fig. 334. It will be seen that 

 it consists of a flat cylinder of glass, about three inches in diameter 

 and half an inch thick, with a large chord cut off so that the portion 

 left is somewhat more than a semicircle; the part where the segment 

 is cut is bevelled from above downwards to an angle of 45, and it 

 will be seen that there is a small disc with an aperture in it denoting 

 the centre of the semicircle. This instrument is used as follows : 



The microscope is placed in a vertical position, and the aperto- 

 meter is placed upon the stage with its circular part to the front 

 and the chord to the back. Diffused light, either from sun or lam}), 

 is assumed to be in front and on both sides. (Suppose the lens to 

 be measured is a dry ^j-iiich ; then with a 1-inch eye-piece having a 

 large field, the centre disc with its aperture on the apertometer is 

 brought into focus. The eye-piece and the draw-tube are now 

 removed, leaving the focal arrangement undisturbed, and a lens 



FIG. 334. Abbe's apertometer. 



supplied with the apertometer is screwed into the end of the draw- 

 tube. This lens with the eye-piece in the draw-tube forms a 

 low-power compound microscope. This is now inserted into the body- 

 tube, and the back lens of the objective whose aperture we desire 

 to measure is brought into focus. In the image of the back lens 

 will be seen stretched across, as it were, the image of the circular 

 part of the apertometer. It will appear as a bright band, because 

 the light which enters normally at the surface is reflected by the 

 bevelled |i-irt of the chord in a vertical direction, so that in reality 

 a fan of 180 in air is formed. There are two sliding screens seen 

 on either side of the figure of the apertometer ; they slide on the 

 vert ic;il circular portion of the instrument. The images of these 

 screens c:in be seen in the image of the bright band. These screens 

 xlmuli! iinir be moved so tlml I //fir edges just touch the periphery of 

 lli<- Ii,n-Jc lens. They act, as it were, as a diaphragm to cut the fan 

 and reduce it, so that its angle just equals the aperture of the objec- 

 bive and no more. 



This angle is now determined by the arc of glass between the 



