142 Remarks on Dr. Knfield's Insliiutea 



consistent with the supposition that the distance of tiie plane 

 surface, either from the object, or the lens, remains unalter- 

 ed. Those who will consult Rutherforth's Optics, Ch, 

 VII. will see that this inconsistency has arisen from an at- 

 tempt to blend into one, two propositions of which the con- 

 ditions were diiferent. We will add, although the remark 

 has no relation to the last edition, that the mistake in the 

 statement of the magnifying power of the double micro- 

 scope (prop. 147.) arose from precisely the same source. 

 Rutherforth investigated the two ratios on which the magni- 

 fying power depends in separate propositions, — first sup- 

 posing the eye at the station of the object glass, and then 

 at the limit of distinct vision. In uniting these two propo- 

 sitions into one, Enfield inadvertently retained the condi- 

 tion of the former. 



Prop. 44. " Reflection is caused by the powers of at- 

 traction and repulsion in the reflecting bodies." This pro- 

 position is altered and abridged from the following in Ru- 

 therforth: " Bodies refract and reflect light by one and the 

 same power, differently exercised in difl^erent circumstan- 

 ces." The illustration of this proposition by the original 

 author is an excellent one, considering the state of optical 

 knowledge at the time he wrote ; but in the hands of his 

 abridger, although all the suppositions made by Rutherforth 

 are retained, and we are required to admit that " bodies at- 

 tract those rays which are very near them, and repel those 

 a little farther from them," yet no use is made of the at- 

 tracting surface, and the most interesting part of the propo.- 

 sition, the reflection produced by the second surface of the 

 medium, (in regard to which so much pains had been taken 

 in the previous scholium to exclude other hypotheses,) is 

 entirely omitted. The student is left to wonder why " at- 

 traction" is mentioned in the proposition as having any 

 concern with reflection ; and the identity of action in the 

 medium by which refraction and reflection are produced, is 

 kept out of his sight. 



Prop. 46. Schol. Although perhaps nothing positively 

 erroneous is advanced in tliis scholium concerning Sir Isaac 

 Newton's theory of fits of easy transmission and reflection, 

 we cannot but object to a naked statement of a theory, 

 stripped of all the facts which it was formed to explain, and 

 -made at the same time in so obscure a manner as must hv.~ 



