532
HISTOHY OF PHYSICAL ASTBONOMY.
would form an object-glass for a telescope, capable of producing an imagefree of colour.
In the Philosophical Transactions for 1753, Dollond pointed out thatthe law of dispersion assumed by Euler was inconsistent with the conclu-sion immediately deducible from the experiments of Newton on the sub-ject, and therefore he maintained that the result of his researches couldnot be entitled to any confidence. Soon afterwards, however, Klingen-stierna, an eminent Swedish philosopher, succeeded in demonstrating thatthe result of Newton ’s experiment was at variance with certain acknow-ledged phenomena of refraction, and upon this ground he justly assertedthat it could not be received as the true law of nature.
Dollond , who was endowed with the true spirit of philosophical enquiry,now resolved to satisfy his mind upon the subject by actual experiment.With this view he formed a prismatic vessel of water, the sides of which■were composed of plate-glass, and inserted into it a glass prism, so thatthe water and glass refracted the rays of light in opposite directions.When the angles of the two prisms were so adjusted that the emergentray was parallel to the incident ray, he found that the emergent ray wastinged with different colours, a result directly contrary to that which New-ton obtained in a similar instance. Pursuing his experiment, Dollond found that when the angles of the two prisms bore a certain relation toeach other, the rays of light emerged free of colour, while at the sametime they were deflected from their original course. It appeared, then,that although the refractions of the rays in passing through the twoprisms were uncompensated, their divergences were completely corrected,and the unavoidable conclusion therefore was, that the dispersion of therays of light did not depend simply on the mean angle of refraction, asNewton had supposed, but that it was also influenced to a certain extentby the nature of the refracting substance.
It now occurred to Dollond that a similar effect might be produced bycombining together prisms of glass of different qualities. By a series ofexperiments he discovered that of the various kinds of glass, crown-glassproduced the least dispersion of the rays, and flint-glass the greatest.He moreover found, that if a prism of flint-glass, and one of crown-glass,were so combined as to refract the rays of light in opposite directions,and if the angles of the two prisms were so related to each other that therefraction produced by the flint-glass was to that produced by the crown-glass as two to three, the dispersion*of the rays would be completely cor-rected, and they would emerge from the compound refracting substance atan angle inclined to their original course.
The step to the construction of an object-glass for a telescope whichwould produce an image free of colour was now obvious. Dollond per-ceived that this might be effected by combining a convex lens of crown-glass with a concave lens of flint-glass, the focal distances of the two lensesbeing in the proportion of their dispersive powers. He therefore con-structed object-glasses upon this principle, which were found to producethe effect contemplated, and thus he was enabled to execute telescopes,in which the indistinctness arising from the unequal refrangibility of lightwas completely obviated *. The term achromatic, which has been aptlyapplied to such telescopes, was originally suggested by Dr. Bevis.
* For an account of Dollond ’s experiments relating to the dispersion of light, see Phil.Trans., 1758, p. 733, et seq.