NOTES TO BOOK IX. 505 



in Essex, is said to have been led by the study of the 

 human eye, which he conceived to be achromatic, to con- 

 struct achromatic telescopes as early as 1729. Mr. Hall, 

 however, kept his invention a secret. David Gregory, in 

 his Catoptrics (1713) had suggested that it would perhaps 

 be an improvement of telescopes, if, in imitation of the 

 human eye, the object-glass were composed of different 

 media. Encyc. Brit. art. Optics. 



It is said that Clairaut first discovered the irrationality 

 of the coloured spaces in the spectrum. In consequence 

 of this irrationality, it follows that when two refracting 

 media are so combined as to correct each other's extreme 

 dispersion, (the separation of the red and violet rays,) this 

 first step of correction still leaves a residue of coloration, 

 arising from the unequal dispersion of the intermediate 

 rays (the green, &c.) These outstanding colours, as they 

 were termed by Prof. Robison, form the residual, or 

 secondary spectrum. 



Dr. Blair, by very ingenious devices, succeeded in pro- 

 ducing an object-glass, corrected by a fluid lens, in which 

 this aberration of colour was completely corrected, and 

 which performed wonderfully well. 



The dispersion produced by a prism may be corrected 

 by another prism of the same substance and of a different 

 angle. In this case also there is an irrationality in the 

 coloured spaces, which prevents the correction of colour 

 from being complete ; and hence, a new residuary spec- 

 trum, which has been called the tertiary spectrum, by Sir 

 David Brewster, who first noticed it. 



I have omitted, in the notice of discoveries respecting 

 the spectrum, many remarkable trains of experimental 

 research, and especially the investigations respecting the 



