REMARKS ON BARROW'S EUCLID. 37% 



in the glass, the pencil copying the same parts upon the 

 paper in the lower half. It is evident that no more of 

 this object can be copied at one time than can be seen by 

 looking directly into the glass, of course the whole tree 

 cannot be seen at once, and cannot be copied without 

 shifting the instrument several times, so as to take it by 

 separate pieces, which cannot be seen at one time, conse- 

 quently there is great danger of losing the truth of the 

 whole, while one is employed on each part. 



REMARK by VV. N. 

 IT is certainly the intention and instruction of the inventor Method of 

 of the camera lucida, that the traciug should be made upon ca^'^iucida 

 that part of the paper where the picture and the point of 

 the pencil can both be seen coincident, and not that a copy- 

 should be taken in the manner described by Mr. Sheldrake. 

 This requires an attention to the small stop, which regulates 

 the quantities of light which enter the pupil from the prism, 

 and from the paper in the same direction; but I have not 

 found it difficult to manage the position of the eye, which 

 is the principal circumstance — and this will perhaps be as 

 easily acquired by a few trials, as by any minute descrip- 

 tion of the process, which may be derived from Dr. Wol- 

 laston's paper in the 17th Vol. of our Journal, p. 1. 



VIIL 



Remarks on some of the Definitions and Axioms in Barrow* $ 

 Euclid. In a Letter from "William Saint, Esq. 



To Mr. NICHOLSON. 



SIR, Cromer in Norfolk, 



August 4th, 1809. 



IN reading over, a few days since, the 7th book of the Remarks oa 

 English edition of Dr. Barrow's Euclid, several objections S^J 8 

 occurred to me against some of the definitions and axioms, 

 which I noted down. On reviewing these objections, I 



must 



