s* 



OFX0MP03ITI0N OP LIGHT. 



ed it without success, and others having persuaded them- 

 selves a little too hastily, that they had completely succeed-* 

 ed; as Abhe Nollet, for instance, whose name has been 

 quoted as an authority. 



I should not myself have had the means I long wished, 

 but from the politeness and enlightened assistance of Mr. 

 Tremery. Fortunately his study was provided with every 

 thing necessary ; but I shall first briefly describe the nature 

 of the experiment, and the conditions indispensably requi- 

 site. 

 Nature cf the The business was to repeat the experiment, in which 

 ex ?-"™-" t V Newton obtained a well defined solar spectrum, the breadth 

 of which, by concentrating the pencil of light, was reduced 

 to -fa} or T ' T of its length ; and which consequently exhi- 

 bited the homogeneal rays incomparably more distinct from 

 each other than in the common spectrum. Opt. Book I. 

 Part .1. Exp. 11. 

 Conditions re- I have already hinted above, that the success depends, 

 quisite to its j s ^ j n operating on a pencil of light that is very small be- 

 fore it reaches the prism ; 2dly. in producing by the prism 

 a. considerable dispersion of the coloured rays; and 3dly. 

 in receiving their dissected image on. a plane very distant 

 from the point of the angle of dispersion.. 

 Obstacles to it.. But these three conditions are not of themselves suffi- 

 cient. It is almost impracticable to attain the desired ob- 

 ject by their concurrence, when the rays arrive at first ire 

 parallel directions; still more if they arrive diverging, as 

 they do when a pencil of light is admitted through a simple 

 hole in the window shutter of a dark room ; in which case 

 the .sensible diameter of the sun's disk must occasion a di- 

 vergence of the pencil. There is only one circumstance fa r 

 vonrable therefore, that in which the rays may be rendered 

 convergent, without infringing the preceding conditions. 

 Newton's m«- The only method of doing this did not escape tho sagacity 



thod, f j\f ew ton. He effected it by placing at a considerable 



converging the _ . _ . •"„ ■ , .. , , , . 



pencil by a lens distance from the shatter, and but a little before tho prism, 



of little con- a ] ens f a long focus, which by its position regulated tho 

 distance of the plane on which the spectrum was to be re- 

 ceived. In this way, and by the assistance of some other 

 precaution^ he resolved this grand problem in optics. 



The 



