68 Dr Goring on Monochromatic Light, fyc. 



in the way of examining the dispersion of a lens ; it is as fol- 

 lows : — Place a small globule of quicksilver on a perfectly 

 black ground of some kind or other without polish ; let it be 

 placed on the stage of a microscope, which must be presented 

 towards a window in such a manner that the uncondensed 

 light thereof shall fall freely upon the globule, for the light of 

 a candle may be employed for the same purpose ;* but it must 

 not be condensed by a lens, or mirror, or silver cap, of which 

 latter the globule would form an image. The object is to allow 

 the globule to form a minute picture, either of the candle or 

 of the window, by its own natural action. All we have to do 

 is then to bring it into the focus of the object-glass or lens, 

 whose dispersion we wish to examine, and to observe the phe- 

 nomena presented by it, both in and out of focus. If it has dis- 

 persion, the different prismatic colours will appear according to 

 the order of their refrangibility. I shall give a sample of those 

 visible with a globular object-glass, of half an inch focus, and 

 2-10ths of an inch of aperture,-f- (being what would be com- 

 monly used for a working instrument,) employed with a per- 

 fectly achromatic eye-piece, and the light of a window. 



In focus there is a centrical image of the window, which is 

 surrounded by a disc or halo of green, which is again sur- 

 rounded by a circle of indigo, the whole being contained with- 

 in a border of purple. 



Within the focus are found the following colours : in the 



* The light of the sun may also be employed, which, when allowed to 

 fall freely on the globule, gives a miniature image of the sun, forming 

 a ti-ue artificial star, (the queen of test-objects to the initiated optician,) 

 as the light it affords is more intense than any other, so it exhibits disper- 

 sion in the most vivid and brilliant manner. 



+ This object-glass was made by Mr W. Tulley, who presented it to 

 me several years before Mr Coddington had introduced the subject to the 

 notice of the public. 'It was made by Mr T. in order to procure a pencil of 

 light in the same state from every part of the object, and to render the field 

 of view alike in every part which it no doubt effects. As, however, its 

 aberration is very violent, so as to require a strong concuve to correct it, 

 which, when applied, utterly subverts the equalization of its various pencils, 

 it has no advantage over an equiconvex lens ; he therefore abandoned it. 

 I believe he will not, therefore, be very jealous about the honour of pater- 

 nity in this matter. A good thing has generally half a dozen fathers to it, 

 and in as many different countries a bad thing only one. 



