OF NATURAL PHILOSOPHY. 255 



and offered so little handle for further enquiry, that 

 their examination dropped, as if by common con- 

 sent ; Newton himself resting content with urging 

 strongly the apparent incompatibility of these pro- 

 perties with the Huyghenian doctrine, but without 

 making any attempt to explain them by his own. 



(281.) From the period of Newton's optical dis- 

 coveries to the commencement of the present cen- 

 tury, no great accession to our knowledge of the 

 nature of light was made, if we except one, 

 which, from its invaluable practical application, 

 must ever hold a prominent place in the annals both 

 of art and science : we mean, the discovery of the 

 principle of the achromatic telescope, which ori- 

 ginated in a discussion between the celebrated 

 geometer Euler, Klingenstierna, an eminent Swedish 

 philosopher, and our own countryman, the admirable 

 optician Dollond, on the occasion of certain abstract 

 theoretical investigations of the former, which led 

 him to speculate on its possibility, and which ulti- 

 mately terminated in its complete and happy exe- 

 cution by the latter ; a memorable case in science, 

 though not a singular one, where the speculative 

 geometer in his chamber, apart from the world, 

 and existing among abstractions, has originated 

 views of the noblest practical application.* 



(282.) The explanation which our knowledge of 

 optical laws affords of the mechanism of the eye, and 

 the process by which vision is performed, is as com- 



* There seems no doubt, however, that an achromatic 

 telescope had been constructed by a private amateur, a Mr. 

 Hall, some time before either Euler or Dollond ever thought 

 of it. 



