140 Tief)hj 1.0 Mr. Horn. 



the optic nerve; — but for wliat purpose it is reflected baclv to 

 the middle of the vitreous humour ; how it is, and why it is, 

 rendered caustic ; why it is exhibited there, if it is not to be 

 seen, and what is to see it, if it be not to be understood to be the 

 optic nerve, — he does not attempt to explain. 



Mr. Andrew Horn seems, somehow or otiier, now to be a little 

 ashamed of his newly-discovered idea, that this image painted 

 in the middle of the vitreous humour in caustic, is to be seen Ir/ 

 the optic nerve behind; but the words which follow — " and thus 

 the optic impression and position of the tangible object arc re- 

 conciled," — if they mean any thing, can have no other interpre- 

 tation ; for, to suppose that light is reflected from the optic 

 nerve to form an image in the middle of the vitreous humour, 

 and that then it is to be reflected buck again to the optic nerve, 

 merely for the sake of being sent there ainl back again, would 

 be sending the light on what some would call a sleeveless errand, 

 but I would call it a IVycomhe hoax. 



Instead of explaining these difficulties, Mr. Andrew Horn 

 passes them unnoticed, and begins an harangue about Sir I,>aac 

 Newton's ignorance and his own correcter knowledge, and brings 

 forward a few legerdemain tricks with a prism. Why this ? I 

 was not inquiring about Sir I. Newton's ignorance of light and 

 colours, nor about reds, nor blues, nor holes in window-shutters 1 

 Why then were these things introduced, if not with the design 

 of drawing the attention from the subject in question, which it 

 did not suit Mr. Horn to reply to, for reasons which perhaps it 

 is not difficult to guess ? 



However, upon closer inspection, this theory which l\Ir. An- 

 drew Horn has given of the prismatic spectrum, has some litlle 

 excuse for its intrusion, as it is evidently own brother by the fa- 

 ther's side to caustic reflection, and appears to me to be equally 

 whimsical, unfounded, and unsatisfactory! 



Of this theory the hole of the window-shutter is the ground- 

 work or first mover, its edge repels polarized light, which by that 

 means is dispersed ; which dispersed rays the prism collects, 

 mixes together, and makes colours of them. Unfortunately for 

 the theory, neither the hole nor the window-shutter is necessary ; 

 as the direct parallel rays of the sun, falling upon a side of a 

 prism at a certain angle in open day, will be formed into the 

 coloured spectrum, without any crossing of rays, inversion of 

 images, holes, or window- shutters, having any thing to do in the 

 matter. 



But as I have no wish to hold any controversy with a man 

 who arrogates to himself the privilege of insulting those who . 

 happen not to admit of his caustic reflections, without being 

 convinced of the propriety of them, I shall make no further in- 

 quiries j 



