Chap, i.] Dollond, Martin, and De la Motte. 169 



are far from uninterefting. Thefe phenomena he at- 

 tribute's to the power which the atmofphere poflfeiTes 

 of reflecting the fainter and more refrangible rays of 

 light, the blue, violet, &c. and upon this principle he 

 alfo explained the blue colour of the fky, and fome 

 other phenomena. 



The fame period produced Mr. Dollond's great im- 

 provement in the construction of telefcopes. It coa- 

 fifts in ufing three glafles of different refractive powers, 

 crowo and flint glafs, which correct each other. The 

 great difperfi6n of the rays which the flint-glafs pro- 

 duces, is the effect of the lead, and is in proportion to 

 the quantity of that metal, which is ufed irrrts ccrmpo- 

 fition. Mr. Martin found the refractive powers -of 

 different glalfes to be in proportion to their ipecific 

 gravity. 



Several difcoveries and improvements have been 

 made fince the time of Newton in that branch of optics 

 which relates more immediately to vifion; but thefe, 

 being rather foreign to the chief fubje6t of this chap- 

 ter, I fhall not detail. One difcovery only. I mail men- 

 tion, becaufe it not only is curious in itfel but becaufe 

 it led to the explanation of feveral circumftances relat- 

 ing to vifion. M. De la Motte, a phyfician of Dant- 

 zick, was endeavouring to verify an experiment of 

 Schemer, in which a diftant object appeared multiplied 

 when viewed through feveral holes made with the 

 point of a pin in a card, not further diftant from one 

 another than the diameter of the pupil of the eye; but 

 notwithftanding all his labour, he was unable to fuc- 

 ceed, till a friend happening to call upon him, he de- 

 fired him to make the trial, and it anfwered perfect- 

 ly. This friend was mcrt-fighted ; and when he ap- 

 plied a concave glafs clofe to the card, the object, 



which 



