THE OPTICAL EQUIPMENT 45 



object is in focus produces, in combination with the objective, 

 an image in the plane of the diaphragm in the ocular, this 

 diaphragm being adjustable in position to suit varying camera- 

 lengths. The focussing ot the image of the diaphragm which 

 is projected on to the ground -glass screen of the camera is 

 effected by rotating the top portion B of the ocular, and the 

 amount by which this is moved can be read off on the scale 

 which is in the position C. A reproduction of the scale is 

 shown in the illustration in each case by the side of the 

 respective oculars. 



Messrs. Zeiss supply a tape-measure which may be con- 

 veniently utilised for determining, by measuring the camera- 

 length only, the exact position which this diaphragm should 

 occupy in the ocular, as indicated on the engraved scale. In 

 order to measure the distance from the eye-piece cap to the 

 ground-glass screen it is only necessary to determine the 

 position of the pointer on the graduated scale of the eye-piece. 

 The scale is divided, and reads from to 9 ; each division 

 represents an alteration in distance of 0*5 mm. between 

 the lenses of the ocular. 



For the short-tube or Continental microscope they are made 

 in two powers, magnifying respectively two and four diameters ; 

 and with the No. 2 ocular the shortest camera that can be used 

 is about seven inches. With No. 4 ocular a camera as short as 

 five inches can be used. At a distance of one metre from the 

 eye-piece cap, the field will be about 20 cm. across with a 

 No. 2 ocular, and about 30 cm. with a No. 4. 



With the ten-inch or English tube, these oculars magnify 

 respectively three and six times, and allowance must be made 

 for this in cases where the longer tube is in use. 



