10 



A POPULAR ACCOUNT 



but refer the reader to our history of 

 optics, in which this and many other 

 matters entirely omitted here will be 

 treated at large. 



While examining the two rays into 

 which the incident ray is cleft by the 

 Iceland spar, Huygens discovered one 

 of the facts from which Newton subse- 

 quently deduced the polarisation of light. 

 He found that if these two rays entered 

 a second prism of the same crystal si- 

 milarly placed with respect to the ray, 

 each of the two rays were again re- 

 fracted, according to the same law which 

 regulated their refraction in the first 

 crystal ; but he found that if the second 

 prism were placed at right angles to the 

 first, the laws by which the rays were 

 refracted were interchanged. In every 

 intermediate position of the second crys- 

 tal, each of the two rays from the first 

 crystal were divided into two, so that 

 the whole primitive ray was resolved 

 into four. These complicated pheno- 

 mena Huygens confessed himself unable 

 to solve, and it remained for the genius 

 of Newton to explain the curious and 

 beautiful property of polarisation. 



(14.) Newton was born in 1642, and 

 in 1663 James Gregory published his 

 invention of the reflecting telescope, 

 consisting of two concave mirrors, the 

 larger of which, receiving rays from the 

 object, was perforated at its axis, and 

 opposite to the perforation and facing 

 the larger, the smaller was placed so as 

 to receive the rays reflected from the 

 larger, and reflecting them to bring them 

 to a focus in a tube placed in the per- 

 foration. The image thus formed was 

 viewed with an eye-glass as in the as- 

 tronomical telescope. 



Three years after this Newton in- 

 vented his reflecting telescope, in which 

 he dispensed with the perforation in the 

 principal speculum, thereby preserving 

 the central rays which were most es- 

 sential to the formation of a perfect 

 image. The rays reflected from the 

 great speculum in Newton's telescope 

 were again reflected to an aperture in 

 the side of the tube by a plane reflector 

 placed at an angle of 45 with the axis 

 of the tube. At this aperture they were 

 viewed in the usual way through an eye- 

 glass. 



(15.) In 1665 the work of Francis 

 Maria Grimaldi, an Italian Jesuit, was 

 published, containing his discovery of 

 the inflexion or diffraction of light. He 

 admitted a ray of light through a small 

 hole into a darkened chamber, and ob- 



served that it was diffused into the form 

 of a cone, and that the shadows of 

 bodies placed in this light were larger 

 than they would have been had the 

 light passed without interruption or de- 

 flexion at their edges. Upon more mi- 

 nute and careful inspection, he observed 

 that these shadows were skirted with 

 three coloured fringes, which grew nar- 

 rower as they receded from the body. 

 He also observed, that when the light 

 was strong there were similar streaks of 

 colour within the shadow, which in- 

 creased in number in the same shadow 

 when it was received at a greater dis- 

 tance from the body. From these phe- 

 nomena he concluded that light, in 

 passing the boundaries of an opaque 

 body, was turned from its rectilinear 

 course, an effect which has been called 

 inflexion or diffraction. 



Another experiment instituted by this 

 philosopher indicated a still more ex- 

 traordinary property of light. Through 

 two small apertures he admitted two 

 cones of light into a darkened chamber, 

 the apertures being so placed that the 

 cones did not penetrate one another 

 until they reached a considerable dis- 

 tance from them. Beyond this distance 

 the light was received upon a screen, 

 and it was observed that the part of the 

 screen illuminated by both cones was 

 the least bright, and that its degree of 

 illumination was increased by depriving 

 it of the light of one of the cones, by 

 stopping one of the apertures. Thus 

 it appeared that a body may actually 

 become more obscure by increasing 

 the quantity of light which shines 

 upon it. 



We have in this chapter attempted to 

 give a short sketch of the principal dis- 

 coveries in optics before the time of 

 Newton. In doing this we shall per- 

 haps be accused of having given an un- 

 equal or disproportionate attention to 

 particular topics, and of having touched 

 too slightly upon others. It must how- 

 ever be remembered that our design was 

 not^ to write a history of optics, but to 

 shew the reader how far the way had 

 been paved for Newton, and what ad- 

 vances had been already made in some 

 of those theories which he has the merit 

 of completing ; and on the other hand 

 to shew that on other subjects, such 

 as the unequal refrangibility of light, 

 absolutely nothing was known, nor any 

 advance made, even in the shape of 

 conjecture. 



We shall proceed in the next chapter 



