MICROSCOPIC OBJECTS. 113 



them ; through omitting this I once had a large collec- 

 tion of them consumed by a species of mite which had 

 insinuated itself into the box. 



" I have never been able to see the markings on them 

 with a power much below^ 250, (that is, 4th of an inch 

 focus,) and therefore microscopes of a lower power cannot 

 be expected to show them, except of a very superior 

 quality ; for it must be constantly kept in mind that that 

 instrument is the best which exhibits an object with the 

 least amplification, aU other things being equal. 



"It is also proper to notice, that single magnifiers 

 will resolve them, but not unless considerable attention 

 is paid to their illumination. > Good doublets of suffi- 

 cient power show them readily with Dr. WoUaston's illu- 

 mination ; but they are most easily made out by the simple 

 hght of a candle in the achromatic microscope, if it 

 possess an angle of apertm-e of about fifty degrees, 

 exhibiting aU their dehcate minutiae with precision, 



" It is affirmed by a very acute experimenter of these 

 scales, that ' aU are difficult, and some seem to defy all 

 power of definition.' The latter part of this quotation is 

 perfectly accurate ; but I differ from the former, inasmuch 

 as many specimens, especially the French ones, are very 

 easy, and unworthy the title of proofs ; and as they 

 might be substituted for those I am describing, and thus 

 a common instrument might pass for one of superior ex- 

 cellence, I feel justified in giving this caution. The size of 

 these scales varies from one nine-hundreth to the one hun- 

 dred- and- sixtieth of an inch in length, and as they decrease 



