In 1729, about two years after the death of Sir Isaac, an individual unknown to science broke the spell in which the subject of the spectrum had been so singularly bound. Mr. Chester More Hall, of More Hall in Essex, while studying the mechanism of the human eye, was led to suppose that telescopes might be improved by a combination of lenses of different refractive powers, and he actually completed several object-glasses upon this principle. The steps by which he arrived at such a construction have not been recorded; but it is obvious that he must have discovered what escaped the sagacity of Newton, that prisms made of different kinds of glass produced different degrees of separation of the _red_ and _violet_ rays, or gave spectra of different lengths when the refraction of the middle ray of the spectrum was the same. [Illustration: _Fig. 6._] In order to explain how such a property led him to the construction of a _telescope without colour_, or an _achromatic telescope_, let us take a lens LL of _crown_ or _plate_ glass, whose focal length LY is about twelve inches. When the sun’s rays SL, SL fall upon it, the _red_ will be refracted to R, the _yellow_ to Y, and the _violet_ to V. If we now place behind it a concave lens _ll_ of the same glass, and of the same focus or curvature, it will be found, both by experiment and by drawing the refracted rays, according to the rules given in elementary works, that the concave glass _ll_ will refract the rays LR, LR into LS′, LS′, and the rays LV, LV into LS′, LS′ free of all colour; but as these rays will be parallel, the two lenses will not have a focus, and consequently cannot form an image so as to be used as the object-glass of a telescope. This is obvious from another consideration; for since the curvatures of the convex and concave lenses are the same, the two put together will be exactly the same as if they were formed out of a single piece of glass, having parallel surfaces like a watch-glass, so that the parallel rays of light SL, SL will pass on in the same direction LS′, LS′ affected by equal and opposite refractions as in a piece of plane glass. Now, since the convex lens LL separated the white light SL, SL into its component coloured rays, LV, LV being the extreme violet, and LR, LR the extreme red; it follows that a similar concave lens of the same glass is capable of uniting into white light LS′, LS′ rays, as much separated as LV, LR are. Consequently, if we take a concave lens _ll_ of the same, or of a greater refractive power than the convex one, and having the power of uniting rays farther separated than LV, LR are, a less concavity in the lens _ll_ will be sufficient to unite the rays LV, LR into a white ray LS′; but as the lens _ll_ is now less concave than the lens LL is convex, the concavity will predominate, and the uncoloured rays LS′, LS′ will no longer be parallel, but will converge to some point O, where they will form a colourless or achromatic image of the sun.