Testing Objectives 71 



was due entirely to a lack of that perception of microscopical 

 detail which can only be acquired by intimacy with objec- 

 tives and their qualities. Especially is this true in lenses 

 of the highest grade. We propose, therefore, to give a few 

 hints which, if not of so much use in the initial stage, may 

 be of aid at a later period. 



FLATNESS OF FIELD. This is an important feature in 

 objectives of low power, and the best method of testing for 

 this quality is either by examining the ruled lines of a 

 stage micrometer or the surface of a piece of ground glass. 

 With the higher powers this feature is sadly neglected, 

 especially in lenses of Continental make, it being advocated 

 by one or two leading workers that it is better to get the 

 utmost perfection of definition in one central point rather 

 than that definition should be sacrificed for flatness of field. 

 It would certainly be a great advantage to the microscopist 

 if the two points could be combined in a more satisfactory 

 manner than at present. English manufacturers in the 

 high powers generally provide a flatter field than their 

 Continental contemporaries. 



COLOUR. Dr. Carpenter's old test for achromatism the 

 examination of the cells in a thin section of deal will give 

 a very good idea of the colour corrections of objectives. 

 For high powers, the markings on a frustule of the diatom 

 * Pleurosigma formosum ' are an excellent test. With the 

 apochromatic objectives these come out quite black and 

 white, while with those of the achromatic series any out- 

 standing colour is at once revealed. Another method is 

 the mercury test adopted by opticians. A small globule of 

 mercury is placed on a slip of ebonite, and a piece of whale- 

 bone or watch-spring is made to snap on it, causing the 

 globule to split up into numerous particles of exceedingly 

 minute size. These globules are then examined with the 

 objective, and can be illuminated by means of a bare gas- 

 jet, lamp, or daylight. Outstanding colour will be re- 

 vealed by the globules. 



