f 



Dr. Goring on Achromatic Microscopes. 425 



under side of the wings of those minute-brown Fig. 6. 

 moths which are found among clothes : Mr. Tulley 

 first discovered them. They are generally mixed 

 with others exactly resembling them in general 

 appearance, but distinguishable from them by their 

 lines being visible with a much smaller angle of 

 aperture, which is always to be regarded as the 

 discriminating test of identity with this class of 

 objects. This feather (a) is about -j^^^^ of an inch 

 long by T^j^ broad. In viewing these objects 

 with single lenses, it must not be forgotten that 

 their aperture is regulated by the dimensions of 

 the hole in the anterior part of their setting, 

 or that which comes next the eye, which must be some- 

 where about equal to their focus to enable them to demon- 

 strate these lines ; that is to say, the cylindrule of light 

 which proceeds from them, measured by the dynameter, 

 must be -^\ of an inch in diameter, if such be the focus 

 of the lens employed. The size of the hole in the 

 exterior part or cell may be much smaller than that of the 

 internal without affecting the dimensions of the said cylin- 

 der from the diverging nature of the rays. It merely, in- 

 deed, determines the size of the field of view. 



Fig. 7. The public has been long acquainted Fiff. 7. 



with the scales of the diamond beetle as a beau- 

 tiful opaque object, without being aware of 

 the valuable occult properties they possess as 

 test objects. The mere true definition of the 

 outline of one of these brilliant bodies on its 

 dark ground, as an opaque object, by a microscope of large 

 aperture, is a pretty good security for its quality. The lines, 

 however, when seen upon them, as represented in the plate, are 

 infallible evidence of a very considerable and effective aper- 

 ture, combined with the requisite distinctness. They require 

 precisely the same management of the light to be seen, both as 

 opaque and transparent objects. When intended to be viewed 

 by intercepted light, they must of course be scraped off and 

 placed between talcs. They are not, when treated in this way, 

 so severe a test as when seen by radiated light, but still excel- 



APRIL— JUNE, 1827. 2F 



