Dr. Goring on Achromatic Microscopes, 431 



may be,, an experienced eye will always be able to see the un- 

 corrected achromatic aberration by this test. Small holes 

 drilled in a thin metallic plate and blacked, form a similar 

 object, and generally all dark bodies seen against intercepted 

 light show the same phenomena. 



A very valuable object to the working optician Is an arti- 

 ficial star ; this may be made by a very minute glass globule 

 stuck on a black ground, or by some grains of platina fused 

 by electricity. Mr. Lister first employed globules of quick- 

 silver for the same purpose. 



His way of procuring them Is, I believe, as follows : — 



Let a small quantity of mercury be squeezed through 

 leather, to clean it ; then put it into a glass tube with some 

 water having a very little gum dissolved in it ; agitate it till 

 it resolves itself into very minute globules ; extract a few of 

 them, and let them be secured on some blacked paper with 

 the help of a little thin gum- water ; they may then be crushed 

 with the finger so as to be totally invisible to the naked eye : 

 the smaller they are the better. 



The light of the sun falling upon them produces a beautiful 

 disk, but common day or candle light does pretty well also. 

 The initiated well know how to appreciate the use and value 

 of this object, which is, perhaps, the best which can be em- 

 ployed to verify the state of the figure, achromatism, cen- 

 treing, grinding, and adjustment, &c. of the small aplanatlcs. 

 All their defects may be clearly looked into and probed by 

 the means of this single object, and the proper remedies ap- 

 plied to them, which, of course, cannot be done unless the 

 nature of the disease is duly ascertained. 



Fig. 12 is the foot of a blue-bottle fly, an interesting 

 opaque object, and an admirable test, its minutiae requiring a 

 great deal of good distinct light, combined with considerable 

 power to become duly manifest. The specimen I have drawn 

 is, I believe, one of the hinder feet, for the three pair all 

 differ somewhat from each other in the proportion of their 

 component parts, though alike in their general structure : (as 

 one of the claws was buried in the gum which was used to 

 stick the foot on the black surface to which it was applied, 

 I have not represented it.) I lament moreover that my draw- 



