NATURAL THEOLOGY. 49 



the most celebrated artists have been forestalled and 

 outdone in what they supposed to be their own dis- 

 coveries and inventions. 



T. There is another imperfection to which teles- 

 cope glasses are subject, and it has occasioned much 

 trouble and perplexity to the makers of these instru- 

 ments, but which affords new reason to admire the 

 hand of a perfect Artist in the construction of the 

 eye. 



A. The difficulty has been, and it still remains 

 an insuperable difficulty, to make any magnifying glas- 

 ses, of the usual shape, act equally in every part. 

 The rays of light which pass through near the edge, 

 will come to a. focus, and form the little circle or im- 

 age, before those do which pass through the middle 

 of the glass ; and so in proportion, at every inter- 

 mediate point. This makes numerous images ; and 

 although the different images are all sufficiently uni- 

 ted in one, for common purposes, yet, in glasses 

 intended for very distinct and accurate vision, it 

 has been found a very serious inconvenience. It 

 is found to be owing to the form of the glass, as the 

 form commonly is, which is that of a spectacle glass. 

 This form, it can be demonstrated, has less power at 

 the middle than at the more distant parts of the glass. 

 The attempt has been made, by changing the form, to 

 get rid of this difficulty ; and by great skill and atten- 

 tion, the improvement perhaps can be effected, Sir 

 Isaac Newton doubted whether it ever could be per- 

 fectly effected. But there is another remedy, though 



