ON NEWTON'S OPTICS. 53 



spectrum in the position of minimum deA'iatiou on a white paper screen and examine it. 

 This method I followed closely, letting the light pass through the prism as near the 

 refracting edge as possible. The sunlight was thrown on the slit by a heliostat worked 

 by the hand (the "porte-lumière"' of Duboscq). The slit was one of variable width 

 belougiug to the Duboscq collection of apparatus. 



Experiments ivith slit and object- glasses of telescopes, etc. — I was naturally surprised to 

 find that it was absolutely impossible to overlook the lines even when the slit was opened 

 to the widest extent that Newton mentions. The number seen at any one time varied 

 according to the prism or lens used or the brightness of the day, or the width of the slit, 

 but they were always plainly visible on the spectrum. One bright day, when the width 

 of the slit was about \ mm., I counted thirty-eight distinct lines, without reckoning 

 others which were vague in outline. They were distinct enough to be visible to half a 

 dozen persons or more at the same time. Afterwards, opeuiug the slit to one-tenth of an 

 inch (the widest used by Newton), I saw plainly ten dark lines on the white paper screen. 

 I ought to say that I was careful always to find the exact distance at which they were 

 best defined, but I did not take any special pains to exclude foreign light, finding that 

 the darkness sufficient for lecture i^urposes was quite enough for all I wanted. I made 

 experiments with three diftereut prisms, viz., one by Duboscq for projection experiments, 

 another belonging to a Duboscq spectroscope, the third was very inferior in its action to 

 either of these. I also used three different lenses — one belonging to a Dollond telescope, 

 of three feet six inches focal length ; the second belonged also to a telescope of somewhat 

 greater focal length ; the third was simply the Duboscq lens used for projection experi- 

 ments. 



On seeing the resiilts, I came to the conclusion at once that it was exceedingly impro- 

 bable that they had not been published before, although I had found no mention of them 

 in any English work that I had been able to consult (nor have I yet) ; nor had I found 

 any allusion to them in Jamin's "Traite de Physique" (1881), nor in Daguin's (1862), 

 although on re-examining this I found something like the experiments, tivo slits, however, 

 being used. But on examining Pouillet (vol. ii, p. 208, 1853), there I found this method 

 recommended and connected with Newton's name. In an earlier French work (Lamé, 

 1840) the same method is recommended, but nothing is said about Newton. 



Circular hole. — In Experiment 11 Newton used a circular hole of one-tenth of an inch 

 diameter. Nothing is said of experimenting with this in the above manner in any of the 

 books I have referred to, but on examining the spectrum due to it and formed in this way 

 I saw /(>«»• lines very distinctly. 



The above experiments, conducted after Newton's method and showing that it gave 

 a spectrum pure enough to show as many as thirty-eight lines, were nevertheless not 

 conducted under a condition by which Newton was restricted. I think it has been 

 sometimes forgotten by writers on this subject that New'ton had no achromatic lens, and 

 that he could not, if he would, have made all the rays fall parallel on the prism by means 

 of a collimating lens. In Experiment 11 he used several different lenses, as may be seen 

 from the extract given above. The dispersion produced by any of them was probably 

 great enough to prevent the appearance of dark lines. It seems probable that the same 

 error which led him to despair of the construction of an achromatic lens did, as another 



