Dr. Goring on Achromatic Microscopes, 429 



power required to show it well must be equal to ^V ^^ an 

 inch ; a very strong artificial illumination is also requisite : 

 it does not show well upon a dark ground, but on one some- 

 what transparent, for which reason a little intercepted light 

 behind it is useful. When treated in this way, the coil may 

 be seen verified, and as I have drawn it, as a transparent 

 object, day-light shows it better than lamp or candle light. 



Fig. 10. Mouse's hair ; — another object of pig. 10. 

 the same class : a, opaque, 6, transparent. 

 Both their varieties occur in the same ani- 

 mal, as represented. The large hairs are 

 punctuated with circular indentations passing 

 spirally round them : they may be distinctly 

 demonstrated as opaque objects. The dif- 

 ficult point is to see them so verified by oblique illumination 

 that they appear like little cups shaded as they are when 

 exposed to the same light : a black ground is absolutely ne- 

 cessary to bring out their structure satisfactorily. They may 

 also be seen very well, as transparent objects, by ordinary 

 compound microscopes with the usual small angles of aper- 

 ture ; but when seen distinctly with a large aperture they 

 are a good test : achromatics will sometimes show the whole 

 tribe of lines without being able to make them out. b is the 

 small mouse hair, seen as a transparent object ; it is che- 

 quered by circular annuli, between which are to be seen 

 longitudinal fibres, which last are the test, not being visible 

 unless by a great deal of good distinct light. Mr. Cuthbert 

 first discovered them with an Amician reflector, which, if 

 really good, shows them better than any other microscope 

 will. 



Fig. 11 is merely a magnified piece of watch plate, with 

 letters enamelled white on a black ground. It is Mr. Tul- 

 ley's favourite object for working all his object-glasses ; by 

 this he judges of the state of their achromatic and spherical 

 aberration, adjustments, &c. Considered as a mere micro- 

 scopic object, it is an exceedingly easy one, being visible to 

 the eye ; any common compound microscope will of course 

 show it fast enough, if its object-glass is cut off and reduced 

 in aperture till its aberration becomes insensible. The diffi- 



