70 Dr Goring on Monochromatic Light, #c. 



merits, are, in his opinion, highly ornamental, and not at all 

 injurious to vision, I can only say I wish him a hetter taste. I 

 have seen people in the streets paying their pence very cheerfully 

 to a peripatetic astronomer for the sight of a beautiful rainbow 

 about the moon, which he had discovered with the curious 

 telescope he was exhibiting, and which I had certainly the 

 honour of seeing myself, together with the beautifully prisma- 

 tic tails of some aquatic insects shown by a microscope con- 

 structed on the same principle as the telescope. If people like 

 to eat dirt they shall meet with no opposition from me, — much 

 good may it do them. 



I have certainly myself seen a thirty inch telescope of two 

 inches and a half aperture in which there was an immense quan- 

 tity of uncorrected colour, nevertheless able to show the star 

 epsilon Bootis very well, and other things in proportion, ow- 

 ing to its spherical aberration being very exactly corrected. 



The only case, I think, in which colour can be said to be cor- 

 rected by an eye-piece, is when an object-glass over-corrected 

 for colour is made to act with a chromatic eye-piece, as I have 

 indeed before stated, having once caused the object-glass of an 

 engyscope to be over-corrected, because it was to act with an 

 erecting eye-piece of the ordinary construction. The beneficial 

 effect of this arrangement was very sensible, and it might either 

 be said that the eye-piece corrected the object-glass, or the ob- 

 ject-glass the eye-piece. I am of opinion that the chromatic 

 aberration of simple microscopes and doublets is not sufficient 

 to produce any sensible detriment to vision, unless with exceed- 

 ingly minute and delicate objects, beyond that of giving a false 

 colouring, especially if oblique light is used. This, however, we 

 soon learn to allow for ; it is moreover considerably concealed 

 by their spherical aberrations. 



As to the spherical aberration of object-glasses and metals, 

 inasmuch as it is sensible to vision, I am persuaded from re- 

 iterated experiments, that the power which an eye-piece pos- 

 sesses of modifying or altering it is exceedingly feeble. A case, 

 however, in which I can be positive that it does actually correct 

 it, is in that of the Gregorian telescope, in which, if the large 

 metal is nearly right in point of figure, for example, a little too 

 much inclined to be spherical or parabolical, the error may be 



