Dr Goring on Monochromatic Light, fyc. 6*9 



centre of the globule is a round spot of purple surrounded 

 by a halo of blue, a second of green, a third of yellow, a fourth 

 of orange, and a fifth of red. 



Without or beyond the focus, a disc of faint greenish yel- 

 low, surrounded by a halo of strong green, without which is a 

 halo of blue, and lastly, one of purple. 



Moreover, when an instrument is perfectly achromatic, there 

 is a total absence of all minute fringes of colour. When it is 

 used on transparent objects with oblique candle-light, nothing, 

 in short, but the natural colour of the object can be made to 

 appear under any circumstances whatever ; for the secondary 

 spectrum is too faint to be seen, unless perhaps in object-glasses 

 of at least an inch aperture. If I had employed for these expe- 

 riments an object-glass of an inch focus, and 4-10ths of an inch 

 aperture, the colours enumerated would have been much more 

 intense and striking ; and if one only of l-8th cf an inch focus, 

 and an equal angle of aperture, far fainter, being nearly in the 

 ratio of the quantity of light dispersed by the object-glass. 



A curious effect is produced by using an object-glass with 

 a very large angular aperture ; for example, by employing the 

 naked opening of the aforesaid globule, which is half an inch, so 

 strong a fog is occasioned by its spherical aberration, as to 

 drown and confuse the chromatic, so that it appears in a great 

 measure obliterated, or very much reduced in quantity, (though 

 there is no doubt that it must be increased.) Chromatic aber- 

 ration is never so sensible as when uncombined with spherical, as 

 in Mr Herschers compound aplanatic lens of one kind of glass, 

 which, having no spherical aberration with a very considerable 

 aperture, is so clear as to allow the chromatic to be seen in 

 the highest perfection.* 



Now it is scarcely necessary for me to ask, whether it is 

 right or not to correct the dispersion I have pointed out ; but 

 how is it to be done by an achromatic eye-piece ? If a man 

 chooses to tell me that those little rainbows and fringes of co- 

 lour, which appear in objects viewed by achromatic instru- 



• Though the dispersion of a diamond or sapphire lens, especially the 

 latter, is much less than that of glass, lenses formed of these gems al- 

 w;i)s Beem to me to colour more than glass, which is accounted for by their 

 greater distinctness. 



