OP THE MICROSCOPE. 85 



false ? la some departments, the most earnest and long-con- 

 tinued discussions have been maintained in regard to the ac- 

 curacy or inaccuracy of certain images as seen by professional 

 microscopists, and, strange to say, these disputes affect the 

 very tests most commonly used, viz., the Podura scale and the 

 test diatoms. 



Makers of objectives, and skilful microscopists, being aware 

 of the fallacies which beset examinations of this kind, resort to 

 certain artificial standards of which the construction is posi- 

 tively known, and which should therefore give appearances 

 conforming to this known structure. Numerous tests of 

 this kind have been suggested, but the only ones generally 

 accepted are the artificial star* and ruled glass plates. Of the 

 latter, ordinary micrometers answer a very good purpose, but 

 the most delicate tests are the famous ruled plates of M. Nobert. 



In the examination of objectives, there are a few simple gene- 

 ral rules which must be observed by the microscopist if he 

 would secure accurate results. 



The first important point, and one to which sufficient attention 

 is not generally given, is the health of the observer at the time of 

 making the trial. The eye is a very delicate organ, and the 

 slightest derangement of the stomach or nerves affects it to an 

 extent that few persons realize. We have an object-glass of 

 comparatively low power, with which, when in good personal 

 health, we find no difficulty in resolving the P. angulatum, 

 though a very slight disturbance of the digestive organs, ren- 

 ders the lines perfectly invisible. 



It must also be remembered that in the case of such delicate 

 observations, personal peculiarities, irrespective of health or 

 sickness, exert a marked influence, so that it does not follow 

 that what one observer sees, all can see. We have frequently 



*Tke artificial star is a very minute globule of mercury, obtained by 

 crushing a small drop by means of a smart tap with a flat slip of iron or 

 ivory. This globule is made to act as a small convex mirror, reflecting 

 the light of a lamp, candle or -window. It is not mentioned by modern 

 writers on the microscope (Carpenter, Hogg, Beale, Frey, etc.), but is 

 used by some of our best opticians. Dr. Royston Piggott, has recently 

 revived its use. Goring devoted considerable apace to an account of the 

 uest methods of using it. 



