OP THE MICROSCOPE. 91 



which lie calls " the simplest and most convenient. " We ven- 

 ture to say, that none but an expert can obtain by it results 

 that are anywhere near accurate, especially with high powers. 

 We therefore consider that any directions upon this subject, 

 addressed to beginners, would be worse than useless. 



Since, however, the resolving power of an objective depends 

 in a large measure upon its angular aperture, we may feel 

 pretty certain that an objective which fails to resolve tests 

 suited to its magnifying power, is deficient in angular aperture, 

 unless, indeed, its inefficiency should arise from want of defining 

 power, which may be tested by other means. Of ordinary work- 

 ing lenses, the half inch ought easily to resolve the Pleurosigma 

 Balticum; the quarter inch should resolve the P. angulatum by 

 oblique light, and those of a fifth or sixth inch focus should 

 resolve the latter test by axial or central light. An eighth, 

 tenth, or twelfth, ought to resolve all the diatoms on the Probe 

 Platte below the 17th. It is true that objectives of a quarter 

 inch focus have been made to resolve everything on the Probe 

 Platte, but such glasses cost too much to render it likely that 

 they will fall into the hands of ordinary students. Twelfths 

 and sixteenths should go through the Probe Platte easily. If 

 they cannot do this it would be better to take a lower power of 

 better quality, and use it with a higher eye-piece. 



We must also be on our guard against an old source of error 

 the use of lined tests which vary from the accepted standard. * 

 Great differences exist in the different specimens of the var- 

 ious test objects that are used, some, owing to individual 

 characteristics and the methods employed in mounting, being 

 much more easily resolved or shown than others. Conse- 



*" The proof objects [finely lined insect scales] originally discovered by 

 me, are sufficient for that purpose in honest hands, and when used with (.he 

 precautions I have pointed out. But it is well known that they have been 

 shamefully abused, owing to the various facilities of resolution which 

 exist between different specimens of lined objects, the external charac- 

 ters of which closely resemble each other; so that it may be said that 

 there are proof objects to suit the capacities of att microscopes; nay, they 

 are actually perverted to the purpose of deceiving the unscientific part of 

 the public in a much more effectual manner, than could possibly have 

 been done without them." Goring. What is true of the scales used by 

 G-oiing is also true, though perhaps not to the same extent, of diatoms. 



