14 



A POPULAR ACCOUNT 



same distance behind the lens. This 

 screen was slowly moved to and from 

 the lens, until that position rr was ob- 

 tained, in which the black lines upon red 

 were seen with the greatest distinctness 

 depicted in the image on the screen. 

 This was evidently the focus of the red 

 rays, because the black lines became 

 distinctly visible only when the red boun- 

 dary was precisely defined. In this po- 

 sition the black lines on the blue part of 

 the image were faint, confused, and 

 scarcely distinguishable. The screen 

 was then moved slowly towards the lens. 

 As it moved the lines on the red part 

 became faint and confused, while those 

 on the blue part became clear and dis- 

 tinct, being found to be as distinct when 

 the screen was removed an inch and a 

 half nearer to the lens, as the lines on 

 the red had been in the first position ; 

 while, on the other hand, the lines upon 

 the red part were scarcely observable. 

 The conclusion was irresistible; the 

 same lens placed at the same distance 

 from red and blue surfaces illuminated 

 by the same candle, brought the rays 

 from the one surface to a focus nearer 

 than those from the other ; and, there- 

 fore, had a greater refracting power on 

 the blue rays than on the red ; which 

 conclusion harmonized exactly with the 

 results of the prismatic experiments pre- 

 viously instituted. 



(20.) Notwithstanding the very con- 

 clusive nature of these experiments, 

 Newton reasons from them with a de- 

 gree of caution and circumspection truly 

 philosophical. " From these experi- 

 ments," he says, " it follows not that 

 all the light of the blue is more refran- 

 gible than all the light of the red ; for 

 both lights are mixed with rays differ- 

 ently refrangible, so that in the red there 

 are some rays not less refrangible than 

 in the blue, and in the blue there are 

 some rays not more refrangible than in 

 the red ; but these rays, in proportion to 

 the whole light, are but few, and serve 

 to diminish the event of the experiment, 

 but are not able to destroy it : for if the 

 red and the blue colours were more 

 dilute and weak, the distance of the 

 images would be less than an inch and 

 an half ; and if they were more intense 

 and full, that image would be greater, as 

 will appear hereafter." 



These experiments were conclusive 

 respecting light which proceeded from 

 the colours of natural bodies. But it 

 still remained to analyze the direct solar 

 light, and to determine the nature of the 



beams of white light, which are the 

 means of rendering all coloured objects 

 visible, and of causing them to transmit 

 the coloured light which emerges from 

 them. 



(21.) If a ray of light, direct from the 

 sun, be admitted through a small aper- 

 ture A, fig. 11, in the window-shutter of 



Fig. 11. 



a dark chamber, a circular image of the 

 sun will be formed by the rays admitted 

 through the hole, and may be received 

 upon a paper screen, as at S S'. We 

 shall, for the present, suppose the hole 

 to be so small, that its diameter may be 

 neglected, and it may be regarded as a 

 physical point. The rays which proceed 

 from the several points of the sun's 

 disc entering the aperture A, cross 

 each other, and that from the highest 

 point proceeds to S' the lowest point 

 of the image, while that from the 

 lowest proceeds to S the highest point 

 of the image. In like manner the ray 

 from the right-hand side of the sun 

 proceeds to the left-hand side of the 

 image, and that from the left-hand side 

 to the right-hand side of the image. In 

 the same way every point of the sun's 

 disc is referred to that point of the image 

 diametrically opposite in position. The 

 image is therefore inverted in whatever 

 way it be considered with reference to 

 the sun. 



The magnitude of the image, or illu- 

 minated spot, on the screen, evidently 

 depends on the distance of the screen 

 from the aperture A, increasing as that 

 distance increases ; and the diameter of 

 this image subtends at the hole A the 

 same angle as the sun subtends at it, or 

 as the apparent diameter of the sun. 



We have here supposed that the hole 

 has no sensible magnitude, or is a phy- 

 sical point. If this be not the case, and, 

 on the other hand, the aperture have a 

 sensible diameter, all that we have above 

 stated will be true of every separate 

 point in it : so that there will be innu- 

 merable images of the sun ; the centres 

 of which will be diffused over a space 



