430 Dr. Goring on Achromatic Microscopes. 



culty, however, consists in showing it with a large aperture 

 in the same way as drawn at a, with the black and white 

 distinctly separated from each other. If there is any spheri- 



Fig. 11. 



cal aberration about the object-glass, it shows itself in the 

 shape of a nebulosity about the confines of the black and 

 white, as at b ; or if worse still, in a diffused nebulosity or 

 fog over the whole of the plate, as at c. The achromatic 

 aberration also shows itself in a fringe of colour mixed up 

 with the fog or nebulous border. There is frequently another 

 appearance which I have not represented ; it consists in a sort 

 of penumbra or double image which cannot be made to lap 

 over the other by any adjustment of the focus, but still shows 

 itself in a sharp but faint margin to the true outline of the 

 figures : reflecting microscopes, if their metals are not truly 

 figured, are very apt to show this flaw in performance. 



We may rest assured that if any object-glass or metal de- 

 fines this object completely well, with such an aperture as will 

 also show the lines on the feathers, it must be good. An 

 objective-glass of 0.933 focus ought to be able to do this with 

 an aperture of 0.38, or even 0.4, if in good adjustment; 

 others of shorter foci with a proportional angle at least. Mr. 

 Cuthbert's metals of 0.3 focus will define it with an aperture 

 of 0.2, which no single aplanatic will ever be made to do. A 

 very excellent proof object for achromaticity may be made of a 

 piece of fine wire-sieve (if dirty so much the better), treated 

 with intercepted light thrown obliquely through its interstices. 

 This is sure to surround the wires with coloured fringes ; 

 and by putting them within and without the focus, the various 

 rays will show themselves in the order of their refrangibility ; 

 however contracted the aperture of a common object-glass 



