ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 253 



(5) Microscopical Optics and Manipulation. 



Nitzschia singalensis as a Test-Object for the Highest Powers.* — 

 A. A. C. Eliot MerHn, as the result of examination of a specimen of 

 Nitzschia singalensis mounted in styrax, thinks that AmphipUura 

 peUucida can no longer be regarded as the supreme test for objectives 

 of the highest power. He is of opinion that, as a test for first-rate 

 objectives of the largest aperture, N. singalensis must henceforth rank 

 far above A. pelhicida, and that any failure to resolve its transverse 

 striae with lenses of 1'30 N.A. and upwards will indicate a faulty 

 objective or faulty manipulation. Nevertheless, the test is severe, and 

 when accomplished with axial light requires good eyesight in addition 

 to good optical means. In practice the author has found that the 

 transverse stride of JV. singalensis can be revealed with a good semi- 

 apochromatic objective of 1*33 N.A. by means of axial light and a 

 Gifford screen. His observations gave 115,200 and 116,000 to the inch. 



M. A. Ainslie does not altogether support this view, and is inclined 

 to think that, until specimens are obtainable of N. singalensis mounted 

 in realgar, the resolution of the transverse strise of A. pellucida in that 

 medium will remain the best existing test of the all-round capacities of 

 an oil-immersion objective. 



C6) Miscellaneous. 



Quantitative Microscopy .f — Under this title T. E. Wallis describes 

 experiments whose aim has been to devise a general method of pro- 

 cedure which will give precision to counts made under the Microscope, 

 and so enable one to obtain, by the use of this instrument, quantitative 

 results which will carry conviction in much the same way as do figures 

 based upon ordinary chemical processes. The substances dealt with were 

 certain food materials such as are frequently adulterated. The general 

 statement of his method is as follows : — 1 . Make a mixture of the pure 

 substance with an equal weight of the adulterant whose amount it is 

 desired to determine. The two substances may either be dried at 100° C. 

 or, preferably, used air-dry ; estimate the moisture present and apply the 

 necessary corrections in the calculations. Mix 0*2 grm., or other con- 

 venient weighed quantity of this standard mixture, with • 1 grm. or 

 other suitable amount of Lycopodium and sufficient of the suspending 

 fluid (e.g. mucilage of tragacanth) to produce a liquid of which one drop, 

 when mounted and examined with a ^-in. objective, shall show from 

 ten to twenty Lycopodium spores in each field. In most cases this result 

 will be obtained when the total volume is about 20 c.cm. A drop of the 

 suspension is transferred to a slide by means of a glass rod, and a cover- 

 glass is applied. Count the number of particles of adulterant and of 

 Lycopodium spores in ten fields. Mount a second drop on another 

 slide, and again record the counts. Find, for each set of ten counts, 



* Journ. Quekett Micr. Club. xiii. (1916) pp. 111-6 (1 photo.), 

 t The Analyst (Journal of Public Analysts and other Analytical Chemists), 

 Deo. 1916 ; also as a separate pamphlet, 19 pp. (1 fig.). 



