CHEMICAL EFFECTS OF LIGHT. 295 



pend, I cUnblved fome giiaiacum in alcohol, and after having 

 waflied a card with the tin6lure, I expofed it for fome time to 

 different parts of the common prifmatic fpeflrum, but with- 

 out producing any apparent change. It therefore became ne- 

 ceffary to have recourfe to other expedients for increafing the 

 power of the fpe6lrum. 



Over the furface of a lens 7 inches in diameter, was parted A broad convex 



a circular piece of paper having its radius -^Vr of an inch lefs '^"^ fcejng 

 I . , I . r . , ^ , . . , covered all but 



than that ot the lens. 1 hadconiequently remainmg uncovered a narrow exte- 



a prifmatic annulus, correfponding in the length of its circum- •■'<"■ ""£» S^^^» 

 ,. -r • , , r 11- ■ 1 3t pleafure, ac- 



KTCnce to a prilm 22 niches long, io arranged by its circular cording to the 



form that any one of the colours might at pleafure be brought lo diftance, a focal 

 a focus, or the fpe£trum might be received as a ring of any di-^^^gy^ or on^aa 

 ameter required, by mere variation of the diftance of the lens, annular fpec- 

 At fliort diftances the exterior margin of the fpe6lrum of courfe *'^"'"* '* 

 was red, and the violet within. The focus of brighteft illu- 

 mination was at 2 t| ; at greater diftances the fpe6lrum again 

 became an annulus with its colours in an oppofite order to the 

 preceding, having the violet on the exterior margin. 



With this apparatus the efFeS produced on muriateoffilver Muriate of filver 



is much accelerated. At diftances tliort of 22^ inches a ring "^J^J^^f^ r 



* '^ affected by th-ife 



is produced ; at 22| a circular dark coloured fpot ; and at concentrated 



about 23 inches appears to be the focus of thefe rays, as ihe'^^J'^J" 

 fpot is then fmalleft; at 23f it is larger, at 24-i it again be- 

 comes a ring (haded to the center; andat24-J-, (unlefs tlie 

 paper has been wetted,) the center remains compleally white 

 though ftrongly illuminated. I have not however been able in but not re.ftoitd 

 any fituation to reftore the white colour to muriate of filvcr, ^"^ "^"^ '* 

 after it has once been tinged, however flightly, by expofure to 

 the moit refrangible rays. 



The experiments on guaiacum neverthelefs will prove dif- Guaiacum was 



. , ,• 1 . - ••• r.i r n affected both 



tinclly, that the powers of the two extremities of the fpedrum ^^^^^ 

 are not only different, but oppofite in their chemical effefls. 



A fufficient quantity of paper having been tinged with the 

 foUuion of guaiacum, was cut into fmall pieces, fome of which 

 were expol^ed to the fun-diine til! rendered compleatly green ; 

 the reft were kept confined from the light till taken out for 

 each experiment. 



The firft endeavour was to afcertain the focal diftance of At a ftort focal 

 thofe rays which gave thedeepeft colour in a given time; and tf^^y" 3*^ 

 jt was found to be about 23 inclies diftant from the lens. At fr-tngibie) the 



IllOrtcr "^^'P^'^ereen 



was producedj-^ 



