1 56 Remarhs on Dr. Enfield'' s Institutes 



will recede with an activity which might seem the effect of 

 animation. But on holding the vessel in the light, the se- 

 cret of these motions will be apparent. Each globule will 

 be seen to have a depression around it, which perceptibly ^ex- 

 tends to the distance of more than half an inch. The globules 

 will be seen to rush together, not from any mutual attrac- 

 tion, but because, in doing it, each descends down an inclin- 

 ed plane. Two needles, laid on water and kept parallel to 

 each other, will exhibit similar appearances. 



In the Optics, under Prop. 18. Exps. 22, 23, 55, 56, &c. 

 the term focus, as used to denote the point as if (rom which 

 diverging rays proceed after refraction or reflection, is 

 changed into imaginary radiant. The latter term is doubt- 

 less the most descriptive of the actual condition of the rays, 

 and by some writers is uniformly employed instead of vir- 

 tual or negative focus. But to introduce this distinction 

 increases the complexness of enunciation of several impor- 

 tant theorems which are" already too complex.* It were to 

 be wished, for the sake of these theorems, that we had some 

 term which should merely express the point where the hnes 

 of direction of a pencil of rays meet, before refraction or 

 reflection, — without including the idea of divergency or con- 

 vergency ; and another to denote the same thing after re- 

 fraction or reflection. As long as this is not the case, we 

 are not cenfident that any advantage is gained by changing 

 the denomination of focus, when virtual, to imaginary radi- 

 ant. But if the change is made at all, it ought at least to 

 be carried through. This has not been done by the Editor ; 

 and the consequence is, that several propositions contain an 

 implied error. He has inserted " imaginary radiant" after 

 "focus" in prop. 55 ; but in props. 22, 23, and 56, which 

 equally required a similar addition, and in prop. 54, which 

 required a substitution, neither has been made. Such an 

 addition would, it is true, have rendered the enunciation of 

 some of these propositions exceedingly perplexed; but 

 consistency demanded that it should be done, or that the 

 language of former editions should be left unaltered. f 



* Sucli, in Enfield's Optics, are prop?. 2], 23, 54, 56. 



t It must be admifted that the language nf former editions, in tliis re- 

 spect, was not entirely consistent witii itself. Defs. 8 and 18, and She Scjiol. 

 to dcf. 13, ijeedcd modification. 



