AN ACCURATE METHOD OF OBJECTIVE AND SUBSTAGE 

 CONDENSER TESTING. 



By H. Hartridge, M.A., M.D., 

 Fellow of King's College, Cambridge. 



Freliminary C omviunication. 



The methods of objective testing at present in use do not give 

 quantitative data, and depend to a great degree on the keenness of 

 vision, skill and memory of the observer. 



A method which does not suffer from these defects consists in 

 measuring with a suitable micrometer the position the image pattern 

 when different parts of the objective aperture are used. If the lens 

 is perfect and in correct focus no movement of the image pattern 

 occurs. If there is movement, however, and if the micrometer reading 

 be plotted against the N.A. of the part of the objective aperture in 

 use, then the graph thus obtained shows the aberrations that are 

 present and their amount, 



A suitable method of isolating objective apertures of given N.A. 

 was obtained by moving by means of a graduated micrometer screw 



a slit-sliapsd aperture placed below an oil immersion condenser. The 

 method of calibrating the micrometer in terms of N.A. has been 

 previously described. (1) 



The best eye-piece magnification was found to be about 100 

 diameters; this was obtained by a 2-3 in. objective and a x 10 

 eye-piece. 



The glass plate micrometer proved most suitable for measuring 

 the displacements of the image patterns. 



The method of illumination has already been described. (2) 



The typical graphs obtained for certain aberrations may now be 

 briefly described. 



Centre of Field. 



Perfect lens (a) correct focus: — a straight vertical line; (b) in- 

 correct focus: — a straight inclined line; (c) incorrect tube-length: — 

 S-shaped line. 



Imperfect lens (a) sj^herical aberration: — a sinuous line (not of 

 regular S-shape), and never a straight line; (b) central astigmat- 

 ism : — a different curve in one azimuth to that given by another. 



(i) Hartridge, Journ. Roy. Micro. Soc., 1918, p. 337. 

 (2) Hartridge, Journ. Quekett Micro. Soc, Nov. iqiq. 



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