118 MICROSCOPIC TECHNIQUES 



than on the image of the entrance opening of the monochromator. 

 The diameter of the condenser diaphragm is set at 6 mm. to insure 

 sufficient spectral purity. 



Both photoelectric and photographic recording may be employed. 

 The quartz photoelectric cell is a No. FJ-405 of General Electric Co., 

 and it is mounted 56.5 cm. above the ocular in a large brass cylinder. 

 This cylinder has a quartz window close to which is a frame that 

 holds a series of circular slits. The cylinder is horizontally adjustable 

 so that a slit can be brought into the optical axis of the microscope. 

 The brass cylinder also contains a type D96475 electrometer tube of 

 Western Electric Co., and a 10^" ohm SS white grid resistor. The 

 output from the photocell is connected to the grid of the tube which 

 is included in a Penick amplifier circuit maintained on three storage 

 batteries of large capacity. The amplified current is measured by a 

 Leeds and Northrup type R galvanometer with a scale 1.5 m. from 

 the galvanometer mirror. 



In order to bring the photocell into adjustment in the optical 

 axis of the microscope, the shadow of an ocular cross hair is pro- 

 jected by means of "white" light on the photocell aperture, which 

 is then adjusted until its center and the center of the image conin- 

 cide. The cross hair in a fluorescent finder placed above the ocular 

 is adjusted similarly with "white" light and ultraviolet radiation. 



For the measurement of absorption curves of larger uniform 

 objects, the object is centered in the cross hair of the finder, the con- 

 denser is focused on the plane of the telescope lens, the objective is 

 adjusted to give a sharp image, and the current generated in the 

 photocell is measured. Then the object is moved away so that only 

 the clear slide is in the optical path and another measurement is 

 made. From these data the percentage transmission and the extinc- 

 tion coefficient can be calculated. The measurements are then re- 

 peated at each wavelength chosen. For studies of thyroid colloid, 

 Gersh and Baker (1943) used a 6 mm. objective, lOX ocular, and a 

 photocell aperture of 11.9 mm. With these optics the light transmis- 

 sion was measured through a tissue area of 143 /x- and a volume of 

 2861 /i,^. For measurements on cytoplasm and nucleoli the respective 

 systems were 6 and 1.7 mm. objectives, 14 and lOX oculars, and 

 8.73 mm. photocell aperture in both cases. 



When the measurements are to be made on smaller and less 

 homogeneous objects such as Nissl bodies, a different and more 



