Opticks. Isaac Newton

Rays was now the least of all. By this means I used to form an oblong Image (such as is pt) [in Fig. 23, and 24.] of Circular Images of the Hole, (such as are ag, bh, ci, &c.) and by using a greater or less Hole in the Window-shut, I made the Circular Images ag, bh, ci, &c. of which it was formed, to become greater or less at pleasure, and thereby the Mixture of the Rays in the Image pt to be as much, or as little as I desired.

       Fig. 24.

      Illustration. In the twenty-fourth Figure, F represents the Circular Hole in the Window-shut, MN the Lens, whereby the Image or Species of that Hole is cast distinctly upon a Paper at J, ABC the Prism, whereby the Rays are at their emerging out of the Lens refracted from J towards another Paper at pt, and the round Image at J is turned into an oblong Image pt falling on that other Paper. This Image pt consists of Circles placed one after another in a Rectilinear Order, as was sufficiently explained in the fifth Experiment; and these Circles are equal to the Circle J, and consequently answer in magnitude to the Hole F; and therefore by diminishing that Hole they may be at pleasure diminished, whilst their Centers remain in their Places. By this means I made the Breadth of the Image pt to be forty times, and sometimes sixty or seventy times less than its Length. As for instance, if the Breadth of the Hole F be one tenth of an Inch, and MF the distance of the Lens from the Hole be 12 Feet; and if pB or pM the distance of the Image pt from the Prism or Lens be 10 Feet, and the refracting Angle of the Prism be 62 Degrees, the Breadth of the Image pt will be one twelfth of an Inch, and the Length about six Inches, and therefore the Length to the Breadth as 72 to 1, and by consequence the Light of this Image 71 times less compound than the Sun's direct Light. And Light thus far simple and homogeneal, is sufficient for trying all the Experiments in this Book about simple Light. For the Composition of heterogeneal Rays is in this Light so little, that it is scarce to be discovered and perceiv'd by Sense, except perhaps in the indigo and violet. For these being dark Colours do easily suffer a sensible Allay by that little scattering Light which uses to be refracted irregularly by the Inequalities of the Prism.

      Yet instead of the Circular Hole F, 'tis better to substitute an oblong Hole shaped like a long Parallelogram with its Length parallel to the Prism ABC. For if this Hole be an Inch or two long, and but a tenth or twentieth Part of an Inch broad, or narrower; the Light of the Image pt will be as simple as before, or simpler, and the Image will become much broader, and therefore more fit to have Experiments try'd in its Light than before.

      Instead of this Parallelogram Hole 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. For by this means, if the Axis of the Prism be parallel to the Perpendicular of the Triangle, the Image pt [in Fig. 25.] will now be form'd of equicrural Triangles ag, bh, ci, dk, el, fm, &c. and innumerable other intermediate ones answering to the triangular Hole in Shape and Bigness, and lying one after another in a continual Series between two Parallel Lines af and gm. These Triangles are a little intermingled at their Bases, but not at their Vertices; and therefore the Light on the brighter Side af of the Image, where the Bases of the Triangles are, is a little compounded, but on the darker Side gm is altogether uncompounded, and in all Places between the Sides the Composition is proportional to the distances of the Places from that obscurer Side gm. And having a Spectrum pt of such a Composition, we may try Experiments either in its stronger and less simple Light near the Side af, or in its weaker and simpler Light near the other Side gm, as it shall seem most convenient.