Dr. llencJieVs Reviewer reviewed. 36T 



Kow the thickness of Doctor Herschel's plate of air or 

 wedge at its greatest opening is stated, in his paper, to be 



>-— part of an inch. This wedge was 5'6 inches in length; 



and an easy calculation will show that from the sharp end, 

 forward for three inches, the thickness (increasing from 

 zero all the wav) did not exceed that determined as above 

 by Newton. This, it is presumed, will be sufficient to 

 show that Doctor Herschel's wedge of air was thin enough 

 for his purpose ; and that any individual ring of a secondary 

 set, when seen within three inches from the sharp end, 

 must appear brok-en or interrupted in such parts as let 

 through the rays of its own colour, granting they were in 

 fits of easy transmission at their incidence on the under- 

 most plaie of the wedge. 



But it might have been assumed that 12 rings, instead 

 of 8, may be rendered perceivable by Newton's object- 

 glasses ; in which case the thickness of the plate of air at 



that place will be -,-- parts of an inch, which is greater than 



the greatest opening of Doctor Herschel's wedge. 



Let us now return to the last quotation froniThe Retro- 

 spect ; in order to consider the import of The two lines 

 which follow the asterisk. There we meet, for the first 

 time in the whole course of the strictures, with something 

 having the f<)rm of an argument. The reviewer tries to 

 prove that certain counteractions will take place to prevent 

 the breaks or interruptions of the secondary rino^s from 

 being perceivable, because the wedge of air, in his opinion, 

 is much too thick. But though we certainly should have 

 welcomed any opportunity of philosophical discussion on 

 a topic of this kind, yet we are obliged to declare that, after 

 having repeatedly and carefully endeavoured to catch the 

 meaning of these two lines, they appear to us altogether 

 unintelligible ; and we are persuaded they nuisf be so judcj^ed 

 of by every one conversant about optics. We are, how- 

 ever, far from saying that the reviewer had not in his mind 

 some meaning or other which bore on him, very cogently, 

 in support of such counteractions : but surely he must have 

 thought very highly of his powers of elucidation, if lie really 

 expected in the compass of two lines to enlighten hi* 

 readers, and' establish his conclusions, on a point of this 

 kind. I'ut as these his conclusions, by his own admis- 

 sion, depend on Doct(^r Herschel's wedges being much too 

 thick, the contrary of which has been made evident, it 

 cannot be expected that such suppoi/'.itious and imaginary 



counteractions 



