64 Dr Goring on Monochromatic Light, fyc. 



as a compilation from the writings of our first opticians, it is 

 admitted to be the best publication of the kind at present ex- 

 tant, and wants no feeble testimony of mine in its favour. 



I presume that the most staunch advocate for the dignity 

 of exact science, while he will insist on the vast superiority of 

 its evidence over that afforded by the senses, will never con- 

 tend that it is to run diametrically opposite to their testimony. 

 I shall venture, though well aware of what I am doing, to 

 impugn its infallibility in the case of these theories of achroma- 

 tism of Professor Robison, Professor Airy, and Mr Codding- 

 ton, on the ground that no artist is able to make an achromatic 

 instrument according to them. * 



All that it is in my power to do will be to oppose theories, 

 by what we call, in common language, facts ; for I must state 

 that, when I view the subject theoretically, I am utterly unable 

 to detect the least flaw in the aforesaid theory. It seems im- 

 possible to deny that instruments constructed according to 

 them ought at least to be achromatic. It is not to be expected, 

 indeed, that a poor sciolist like myself should bo able to school 

 men of such highly mathematical acquirements as the afore- 

 said savans : the task must evidently be left to mathematicians 

 whose calibre is still larger than theirs. I have already par- 

 tially expressed my own views concerning achromatism, and I 

 shall endeavour now to expand them into the following proposi- 

 tions, which rest only on the basis of the evidence of the 

 eyes or experience. 



1. When achromatism is obtained by the adjustment of len- 

 ses to particular intervals, as in the case of the Huygenianeye- 



* We surely have a right to form a certain estimate of the soundness of 

 a theory by its fruit?. I once had the honour of looking through a tele- 

 scope of the Gregorian form, but constructed with mirrors of glass by Mr 

 Tulley, according to the theory of Professor Airy. Mr T. had expended 

 the whole of his skill upon the instrument, and had worked the said mirrors 

 over and over again a great number of times till bis patience was exhaust- 

 ed, yet the confusion in the vision produced was so great, tli.it it was but just 

 possible to recognize objects. I think when such an artist as Mr T. can- 

 not bring a telescope something near to distinct vision, when working to 

 the utmost of his ability by the theory laid down for him, there must be 

 something wrong about it. 



