OF NEWTON'S OPTICS. 



placed that O L and 0' L should be 

 equal, which was always possible, the 

 magnitude of the image O' would be 

 exactly equal to that of the hole. This 

 image was therefore considerably less 

 than the illuminated circle cast on 

 the wall or screen without the inter- 

 position of the lens. A prism A B C, 

 placed behind the lens, deflected the 

 rays emerging from it, and formed 

 the oblong spectrum RV, the breadth 

 of which was equal to O', and which 

 was free from any penumbra. The 

 length of the spectrum in these expe- 

 riments is never changed. In the pre- 

 sent experiment he succeeded in reduc- 

 ing the breadth to about one- sixtieth of 

 the length ; and therefore diminished 

 the mixture of heterogeneous lights in 

 any part of it proportionally. Although 

 it was impossible by this method, or 

 perhaps by any other, to obtain a beam 

 of absolutely homogeneous light, yet 

 what was thus obtained was sufficiently 

 simple and homogeneous for all the 

 purposes of experiment. 



Some other ingenious expedients were 

 resorted to for the simplification of light. 

 The diminished breadth of the spectrum, 

 while it gave a purified homogeneous 

 light, gave a very small quantity of it. 

 To obtain it in greater abundance, it 

 occurred to Newton to form a great 

 number of small holes in the shutter, in 

 the same horizontal row, so as to obtain 

 several of these spectra placed parallel 

 to each other, and thus form one broad 

 one, in which the lights should be as 

 homogeneous as in a single one of small 

 breadth. Or what was equivalent, and 

 still more effectual, instead of a row of 

 holes, he formed one narrow slit in the 

 shutter extending in an horizontal direc- 

 tion, so as to admit a thin sheet of light. 

 By this means a spectrum of any required 

 breadth may be formed, in which the 

 light is as pure and homogeneous as in 

 a spectrum formed by light admitted at 

 a round hole, whose diameter is equal to 

 the breadth of the slit. 



Newton suggests another very ingeni- 

 ous means of obtaining a spectrum, in 

 which lights would be supplied of dif- 

 ferent degrees of purity and intensity. 

 Let a narrow triangular hole be cut in 

 the shutter in an horizontal direction, as 

 fig. 28, O o. The sunbeam admitted 

 through this hole will have an edge like 

 a knife. The broadest part O of the ray 

 will form a spectrum RV, in which the 

 intermixture will be in proportion to the 

 base O of the triangle ; but as the ray 



25 



diminishes in breadth towards o, the cor- 



respondingparts of the spectrum, towards 



Fig. 28. 



R V, are increased in the purity of the 

 light, heterogeneous rays being less in- 

 termixed as the breadth of the triangle 

 is diminished, as is evident from the 

 figure. Having a spectrum of this kind, 

 experiments may be tried either in its 

 stronger, though less simple light, on the 

 side RV, or in the weaker and more 

 simple light on the side r v, as may seem 

 more suitable to the objects of the in- 

 vestigation. 



In all experiments on homogeneous 

 light, Newton states that he found it 

 necessary to use great precaution in order 

 to be secure of success. The chamber 

 should be carefully darkened to avoid the 

 disturbance arising from rays of white 

 light scattered casually about. The glass, 

 both of the prisms and lens, should be 

 free from veins, striae, air bubbles, and 

 other inequalities. He found it so dif- 

 ficult to procure good prisms, that he 

 frequently used transparent liquids in- 

 closed in hollow prisms, formed of pieces 

 of plate glass fixed together at proper 

 angles. 



With light simplified by the methods 

 we have now explained, he tried the 

 experiment explained in (26), and 

 found that a pure homogeneous ray ad- 

 mitted of no dilatation by the prism, and 

 therefore concluded that the light of 

 which it was composed was all equally 

 refrangible. With a view of establish- 

 ing the same principle, he took two small 

 circular pieces of while paper, and illumi- 

 nated one with light direct from the sun, 

 and the other with homogeneous light 

 obtained by one of the methods which 



