MICROSCOPIC ILLUSTRATIONS. 271 



cipal condensation of light is not at the focal point, but 

 within it ; beyond the focus it is evidently much fainter 

 than at the focus ; the field of view is manifestly brighter 

 within the focus than anywhere else. Some still more 

 remarkable peculiarities present themselves to the close 

 observer, as the image vanishes both within and without 

 the focus ; thus the object may be put a certain distance 

 beyond the focus without disappearing, which, however, it 

 ultimately does, enveloped in a strong fog ; within the 

 focus it may be said to disappear instantly, but quite 

 in a different manner from what it did beyond the focus, 

 for the white figures swell out at the edges, forming a 

 penumbra, or well-defined border, without any nebulosity 

 about it, (see figure 8 W .) It is evident that there is no 

 diffusion of light within the focus, for the black ground of 

 the enamelled plate appears quite black. 



On applying Sir D. Brewster^s monochromatic lamp, the 

 fringe of prismatic colour, which was mixed up with the 

 foregoing phenomena, disappears, and allows them to be 

 more distinctly seen. 



2d. Artificial Star used with the light of a window. 



The appearances exhibited by the dial plate are con- 

 firmed, and rendered still more striking, by this brilliant 

 object. In focus there is a strong coma surrounding the 

 miniature image of the window seen in the globule, and 

 extending to a considerable distance from it, so that the 

 outline or figure of the globule cannot be seen (figure 70 

 The globule may be put some distance beyond the focus 

 without causing the spectrum of the window to disappear, 

 which it does at last, in a diffused nebulosity, (see figure 7 X/ .) 

 Within the focus the light of the window instantly swells 

 out into a circular disc, the light of which is much more 



