METHOD OF WORKING 161 



Great care must be taken not to overheat the slide, or a darkening in colour is likely to 

 occur and damage is done to the mount. 



The mountant is quite permanent and requires no cement ring around the cover- 

 glass, although such a ring enhances the appearance of the slide. It is easier and quicker 

 to use than either natural or synthetic resins, and permits the use of oil immersion ob- 

 jectives immediately upon cooling. The refractive index is probably between 1-70 

 and 1-75. 



This mountant gave excellent results with clean and strongly siliceous material, but 

 owing to the degree of heat required to melt the crystalline mixture, it could not be used 

 in the examination of fragile solenoid diatoms such as Giiinardia or Leptocylindrus spp., 

 as such treatment was accompanied almost invariably by the collapse of the frustules 

 into an unidentifiable mass. 



Much of the routine work was done, however, by examining the prepared material 

 mounted dry upon the cover-glass. This method was always used in the examination 

 of such genera as Rhizosolenia, Corethron, Giiinardia, etc. 



The film of diatoms, whether as a wet or a dry preparation, was examined with a 

 modern microscope with a built-in mechanical stage and a centring substage focussing 

 by rack-work; it was fitted with an achromatic dry condenser of n.a. i-oo, inter- 

 changeable with an oil-immersion condenser of n.a. 1-40. A battery of apochromatic 

 objectives was used, comprising 16 mm. of n.a. 0-35, 8-0 mm. of n.a. 0-65, 4-0 mm. 

 of N.A. 0-95, and 2-0 mm. of n.a. 1-40. The body of the microscope contained a 2-in. 

 diameter draw-tube fitted with rack-work extension allowing adjustments to be made 

 to accommodate variation in cover-glass thickness. A squared micrometer eyepiece was 

 invariably used. The illumination was provided by a 60-watt pearl electric bulb for low- 

 power routine work, and a tightly wound spiral filament lamp of 12 volts and 36 watts 

 provided illumination for high-power work. The lamp-houses were of special design to 

 prevent interference from extraneous light, and were fitted with iris diaphragms to con- 

 trol the volume of light used. The lamp was placed about 10 in. from the tail mirror of 

 the microscope, and the interposition of light filters was sometimes found to enhance the 

 image. In order that the lists of species should indicate the order of dominance, counts 

 were made by means of the squared eyepiece, and all organisms were counted over as 

 many optical fields as were thought necessary for the correct interpretation of the num- 

 bers obtained. The number of fields counted varied from 20 to 200 fields for each sample. 

 This procedure will obviously afford only a very rough indication of the relative 

 abundance of the different species, and cannot be expected to yield results comparable 

 with those given by the far more laborious methods now employed for quantitative 

 phytoplankton estimation. 



ACKNOWLEDGEMENTS 



Throughout this work recourse has been made to the published accounts of the 

 scientific results obtained by other Antarctic expeditions : in particular to Heiden and 

 Kolbe (1928) on the marine diatoms obtained by the German South Polar Expedition, 



