ON NEWTON'S OPTICS. Bl 



" lu the Sun's Light, let into my darkened Chamber through a small round hole in 

 m}"- Window-shut, at about 10 or 12 feet from the Window, I placed a Lens, by which 

 the image of the hole might be distinctly caht upon a sheet of white Paper, placed at the 

 distance of six, eight, ten or twelve Feet from the Lens. For according to the différence 

 of the Lenses I iised various distances, which I think not worth the while to describe. 

 Then immediately after the Lens I placed a Prism, by which the trajected Light might 

 be refracted either ui")wards or sideways, and thereby the round image which the Lens 

 alone did cast upon the Paper might be drawn otit into a long one with Parallel Sides, 

 as in the Third Experiment." The " oblong " image thus formed he received upon another 

 paper placed by trial "at the just distance where the Rectilinear Sides of the Image 

 became most distinct." Tn this case, he says, "the circular images of the hole extended 

 into one another the least they could." " By using a greater or less hole in the Window- 

 shut " he made " the Circular Images to become greater or less at pleasure," and thereby 

 the " mixture of the Rays in the Image to be as much or as little" as he desired. " By 

 this means," (p. 49) " I made the breadth of the image to be forty times and sometimes 

 sixty or seventy times less than its length." 



" Yet," he goes on to say (p. 49), " instead of the circular hole F 'lis better to substitute 

 an oblong hole shaped like a long Parallelogram, with its length parallel to the Prism. For if 

 this hole be an Inch or two long, and but a tenth or tiventiéth part of an Inch broad or narroiver, 

 the Light of the Image M'ill be as Simple as before or Simpler, and the Image will become 

 much broader, and therefore more fit to have Experiments tried in its Light than before." 



Instead of this "Parallelogram-hole," he says, "may be substituted a Triangular one 

 of equal sides, whose Base, for instance, is about the tenth part of an Inch, and its height 

 an Inch or more." The edge of the prism is, of course, placed parallel to the perpendicu- 

 lar of the triangle. " The Image will now be formed of Equicrural Triangles." — 

 " These triangles are a little intermingled at their Bases but not at their Vertices," and 

 therefore " the light where the Bases of the Triangles are is a little compounded, but on 

 the darker side is altogether uncompounded." 



He is careful in mentioning precautious to be attended to in the experiments — the 

 exclusion of foreign light from the chamber, a good lens, a prism of large angle, " suppose 

 of 70 degrees, and to be well wrought, being made of Glass free from Bubbles and 

 Veins," etc. 



In the above description I have italicized the breadth of the hole, the " twentieth 

 parth of an Inch " " or narrower," because " ^Vth of an inch broad " is the statement which 

 WoUastou makes about the width of the " crevice " which he used when he discovered the 

 dark lines. It is curious that Proctor should have referred to Newton's experiments with 

 the " oblong " aperture and not have noticed that it was narrow enough to be called a 

 "slit." Neither Newton nor Wollastou u-se the term slit themselves, but this term, or 

 rather " a narrow slit," is applied in a description of Newton's experiments given in an 

 account of Newton's optics (64 pages) published in " The Optics," issued in the " Library 

 of Useful Knowledge " (1830). Lloyd, " Light and Vision " (1831) and " Wave Theory of 

 Light," refers correctly to the experiments also, although he does not employ the word 

 " slit." Where the error first crept in I have not the means of determining. 



Wollaston's account of his own discovery is in a paper in the ' Philosophical Trans- 



