146 MICROSCOPIC TECHNIQUES 



5. Quantitative Estimation of Ash 



Attempts at the quantitation of the relative amounts of ash left 

 by various structures were made by Schultz-Brauns (1931) based 

 on a standardized development of photomicrographs. However, this 

 method has many inherent difficulties and can yield little. 



Scott (1933b), and Williams and Scott (1935) developed a photo- 

 electric apparatus to measure the intensity of light reflected from 

 the ash. This light intensity is roughly proportional to, and serves 

 as an approximation of, the quantity of the mineral residue. While 

 the method obviously leaves much to be desired, as Scott would 

 no doubt be the first to admit, it is capable of furnishing some 

 information that at present can be obtained in no other way. 



An idea of the sensitivity of the apparatus, as assembled and used 

 by AVilliams and Scott, may be gained from the fact that, with a 

 magnification of 700X, the ash produced in a 5 ju, section by a single 

 hepatic cell nucleus results in a galvanometer deflection of about 

 half-scale (25 cm.). 



Williams and Scott Photoelectric Apparatus. The microscope 

 illumination is furnished by a 6 volt, 108 watt ribbon filament 

 projection lamp enclosed in a ventilated housing. This lamp is 

 supplied through a constant-voltage regulator and a step-down trans- 

 former. The slide is held by a special mechanical stage which has 

 rack and pinion adjustment laterally and vertically and a fine screw 

 adjustment axially; and the microscope, mounted horizontally, is 

 fitted with a Zeiss aplanatic 1.2 condenser, a Leitz No. 3 objective, 

 various oculars, and a clamped-on 90° reflecting prism. A fixed 

 diaphragm made of a disc slightly larger than the aperture of the 

 objective is fitted into the ring on the condenser lens. The light 

 emerging from the ocular is reflected by the prism to a mirror which 

 in turn reflects it to the photocell. 



The gas-filled photocell is mounted in a light-tight copper box 

 which serves as an electrostatic shield as well. The cover of this 

 box carries rotatable and interchangeable 3 in. white cardboard discs 

 with various size openings to determine the illuminated area on the 

 photocell. A shutter under the disc permits exposure of the photocell 

 when desired. Within the copper box containing the photocell, a 

 rD54 Pliotron tube is mounted with its 10^ ohm high-resistance 

 shunt. The photocell is connected directly to this tube, and the 



