tec 



23g ANSWERS TO OPTICAL QUESTIONS, 



Fafts refpeftlng ther he IS to ufe Mr. Forfyth*s compofition, Mr. Bulingham's 

 ingrafted fruits, boiled lififeed Oil, or my medication. I only maintain that the 

 permanence, wounded parts of trees want fomething to deftroy the infefls 

 and vermin, and heal the wood, from which the trees are kept 

 in health. 



Let thofe who are blefled with fruit-plantations attend to 

 their prefervation, and not leave them to the ilate of unaffifted 

 nature. 



I am, Sir, 

 Your moft obedient Servant, 



Tho. Skip Dyot Bucknall. 

 Hampton-Court, 

 \2th Oa. 1801. 



XIIL 



Explanation of the SuhjeBs concerning which ^uefiions werepro^ 

 pofed at Page 71 of the prefent rolujjie; namely, I, The Place 

 of the Ere6i Image behind a Concave Speculum \ 2. Of the 

 Image formed by a Concavo Convex Mirror, which is not Left- 

 handed, and has the Property cf revolving along with the Mir- 

 , rori and $. The Figure of the Sky. By W.N. 



ilnfwers to ccr- /xS the queftions of my correfpondent R. B. have not been 

 tain qucftions. treated by any other of my friends, I (hall here difcufs them 



according to my promife. 



Ere£l image in ^ • '^^^ queftion refpeding the ered image in the concave 



a concave mir- mirror relates to a phenomenon which formed the fubje6l of 



'**'• difcuffion in print very early in the lafl: century, though I 



cannot at this moment recollefl the authors who have related 



the fa6ls and arguments. R. B. is perfedly corred as to the 



♦arious divergence of the rays compofing the pencils which 



iflue from radiant points at different diftances from the eye, but 



it does not appear that our notions of diftance are greatly re- 



Our judgment of gulated by that circumftance. Our judgment of the diftances 



diftance is not ^f ^ear objeds is partly founded on the adjuftment required to 



founded on the , , . , ■ r \r i i-/,- \-> •/- i i 



nature of the be made m the eye itlelt to produce diftinct vjfion by the rays 

 pencils of light J of the feveral pencils, as R. B. remarks, but much more on 

 a circumftance commonly noticed by authors, but by him over- 

 looked, namely, the degree of convergence between the op- 

 tical axes of the two eyes which is required to avoid fquinting, 



or 



