/ of TICAL ABERRATION. Ij^ 



III. 



On the Aberrations of Light pajfmg through Lenfes, By Mu 

 EzEKiEL Walker. 



A HE difcovery of the aberration of the rays of light, caufed Newton's dif- 

 by their unequal refrangibility, formed a new area in the fci- ^^J^^^^tions of 

 ence of optics. It is the foundation of all Sir Ifaac N'€'ivton's coloixxtdtnyi, 

 difcoveries in light and colours ; and alfo the foundation of 

 mod of the ufefui improvements in the conftrudion of optical 

 inflruments, that have done fo much honour to our country. 

 And this fcience may ftill derive furthetimprovements from 

 tlie fame difcovery, not only in (heconftrudlion of inftruments, 

 but alio in explaining fome curious phenomena in nature. 



But before I attempt to fliow how the ufe of this property 

 of vifion may be extended. It feems neceflgiry to give a (hort 

 account of that kind of aberration which arifes from the un- 

 equal refrangibility of the differently coloured rays of light : ^ 

 the other aberration, or that which is caufed by the fpherical 

 figure of the lens, is not here confide^ed as being inconfidei- 

 able when compared with the former. 



Therefore let A C B, Fig. 2, Plate X. reprefent a piano- Aberration of 

 convex lens ; P A and R B two pencils of white or compound [h/ju^^h a^kn^ 

 rays of light, falling upon it at the points A and B in a direc- 

 tion paiallel to|(|s axis. Alfo A| A xt; and Byv be the red 

 or leaft refrangible rays, and A'^and B ^ x the violet, or moft 

 refrangible. The red ray from A will cut the violet ray from 

 B at the point or, and th« red ray fronj B will cut the violet ray 

 from A at the point^^; through thefe interfe<5lions dl^w the 

 line xj/t and this line will be the diameter of the leafl circular 

 fpace into which all the rays that fall upon the lens, parallel 

 to its axis, can be co!le6led. And this circle, which for bre- 

 vity's fake is called the circle of abeiration, is the true focus * 

 uf the lens or place where the image of the obje6l is formed. 



Let the fine of incidence going oulof giafs be n, the fine;* 

 of refradion (into air) of the leaft ^and the mod refrangible 

 rays he p and (j ; then if a piaoio-convex lens be expofed with 

 the plane fide to the fun, thediameler of the circle of aber- 

 ration xy (or image of the |un formed of rays of different re- 

 fra-ngibility) is to the diameter of the lens A B, as q—pio 

 qxp-Qn* 



* This theoi;em is w£ll known to mathematicians. 



■•vT' From 



