[Illustration:  Fig. 7.]

This was the first analysis of solar light by Newton; but the scientific mind is fond of verification, and never neglects it where it is possible.  Newton completed his proof by synthesis in this way:  The spectrum now before you is produced by a glass prism.  Causing the decomposed beam to pass through a second similar prism, but so placed that the colours are refracted back and reblended, the perfectly white luminous disk is restored.

[Illustration:  Fig. 8.]

In this case, refraction and dispersion are simultaneously abolished.  Are they always so?  Can we have the one without the other?  It was Newton’s conclusion that we could not.  Here he erred, and his error, which he maintained to the end of his life, retarded the progress of optical discovery.  Dollond subsequently proved that by combining two different kinds of glass, the colours can be extinguished, still leaving a residue of refraction, and he employed this residue in the construction of achromatic lenses—­lenses yielding no colour—­which Newton thought an impossibility.  By setting a water-prism—­water contained in a wedge-shaped vessel with glass sides (B, fig. 8)—­in opposition to a wedge of glass (to the right of B), this point can be illustrated before you.  We have first of all the position (dotted) of the unrefracted beam marked upon the screen; then we produce the narrow water-spectrum (W); finally, by introducing a flint-glass prism, we refract the beam back, until the colour disappears (at A).  The image of the slit is now white; but though the dispersion is abolished, there remains a very sensible amount of refraction.

This is the place to illustrate another point bearing upon the instrumental means employed in these lectures.  Bodies differ widely from each other as to their powers of refraction and dispersion.  Note the position of the water-spectrum upon the screen.  Altering in no particular the wedge-shaped vessel, but simply substituting for the water the transparent bisulphide of carbon, you notice how much higher the beam is thrown, and how much richer is the display of colour.  To augment the size of our spectrum we here employ (at L) a slit, instead of a circular aperture.[6]

[Illustration:  Fig. 9.]

The synthesis of white light may be effected in three ways, all of which are worthy of attention:  Here, in the first instance, we have a rich spectrum produced by the decomposition of the beam (from L, fig. 9).  One face of the prism (P) is protected by a diaphragm (not shown in the figure), with a longitudinal slit, through which the beam passes into the prism.  It emerges decomposed at the other side.  I permit the colours to pass through a cylindrical lens (C), which so squeezes them together as to produce upon the screen a sharply defined rectangular image of the longitudinal slit.  In that image the colours are reblended, and it is perfectly white.  Between