METHOD OF EXAMINING DISPERSIVE POWERS, QQ 



If a beam of day-light be admitted into a dark room by aE xper 5 ments by 

 crevice ^ of an inch broad, and received by the eye at thew hich tht '°~ 

 diftance of 10 or 12 feet, through a prifm of flint-glafs, free t0 be four# 

 from veins, held near the eye, the beam is feen to be feparated 

 into the four following colours only, red, yellowifh green, blue, 

 and violet ; in the proportions reprefented in Fig. 3. 



The line A that bounds the red fide of the fpectrum is fome- Remarkable 



what confufed, which feems in part owing; to want of power*"?^ 8 , 1 " 6 !^' 118 t0 

 • .i . , ,- , ,t„ i- n , i the fpe&rum. 



in the eye to converge red light. The line B, between red 



and green, in a certain pofition of the prifm, is perfectly dif- 

 tinct ; fo alfo are D and E, the two limits of violet. But C, 

 the limit of green and blue, is not fo clearly marked as the 

 reft ; and there are alfo, on each fide of this limit, other diftintt 

 dark lines, / and g, either of which, in an imperfect experi- 

 ment, might be miftaken for the boundary of thefe colours. 



The pofition of the prifm in which the colours are mod Spaces occupied 

 clearly divided, is when the incident light makes about equal by thecolours i 

 angles with two of its fides. I then found that the fpaces AB, 

 BC, CD, DE, occupied by them, were nearly as the numbers 

 16, 23, 36, 25. 



Since the proportions of thefe colours to each other have been 

 fuppofed by Dr. Blair to vary according to the medium by 

 which they are produced, I have compared with this appear- 

 ance, the coloured images caufed by prifmatic veiTels contain- 

 ing fubftances fuppofed by him to differ moft in this refpecl, 

 fuch as ftrong but colourlefs nitric acid, rectified oil of turpen- 

 pentine, very pale oil of faflafras, and Canada balfam, alfo 

 nearly colourlefs. With each of thefe, I have found thenearly the fame 

 fame arrangement of thefe four colours, and, in fimilar pofitions invariousbodies » 

 of the prifms, as nearly as I could judge, the fame proportions 

 of them. 



But, when the inclination of any prifm is altered fo as to but is changed 

 increafe the difperfion of the colours, the proportions of them with the incli * 

 to each other are then alfo changed, fo that the fpaces AC and 

 CE, inftead of being as before 39 and 61, may be found altered 

 as far as 42 and 53.* 



H 2 By 



* Although what I have above defcribed comprifes the whole oflnvifible heat 

 the prifmatic fpeclrum that can be rendered vifible, there alfo pafs on [£ aI ang rays on 

 each fide of it other rays, whereof the eye is not fenfible. From re <j.' 

 Dr. Herfchel's experiments (Phil, Tranf. for 1800) we learn, that 



