HISTORY OP PHYSICAL ASTRONOMY.
Altliougli Dollond is entitled to the credit of having executed achro-matic telescopes by the aid of his own researches on the dispersion oflight, there is reason to believe that he was not the first individual whoaccomplished that object. It appears, in fact, that as early as the year1729, Chester More Hall, a gentleman residing in Essex, having reflectedupon the circumstance that the different humours of the eye so refractthe rays of light as to produce images free of colour, ivas led to considerthe possibility of combining together different substances, so as to formobject-glasses for telescopes which would produce a similar effect. Afterdevoting some time to the enquiry, he found that by combining togetherdifferent kinds of glass, the effect of the unequal refrangibility of lightwas corrected; and, in 1733, he succeeded in executing telescopes whichexhibited objects quite free of colour. One of these instruments, althoughits focal length was no more than 20 inches, bore an aperture of 2|inches. More, who was living in independent circumstances, and whoappears to have been nowise anxious about his reputation, did not takeany trouble in communicating his invention to the world *. It has beeninsinuated that Dollond had seen one of More’s telescopes, and that hewas indebted to this circumstance for his construction of the achromaticobject-glass; but the statement does not rest upon any solid foundation,and therefore it can only be regarded as an unfounded calumny.
The achromatic telescopes of Dollond soon became known throughoutthe whole civilised world, and were universally adopted by astronomers asaffording an incontestable advantage over the ancient refracting telescopes.In one respect, however, they laboured under a defect which very muchrestricted the range of their efficiency. This consisted in the impossibi-lity of obtaining pieces of pure flint-glass of sufficient magnitude to servefor telescopes of large aperture. It happened, from this circumstance, thatno astronomical discoveries of importance resulted from the invention ofthe achromatic telescope. Huyghens and Cassini, by their combined ex-ertions in the preceding century, seemed to have so thoroughly scrutinizedthe celestial regions, as to leave little or nothing to be gleaned by theirsuccessors, until instruments should be constructed capable of penetratingstill further into the bosom of space. This remark is applicable to re-flecting as well as refracting telescopes; for it is to be borne in mind, thatnot a single discovery had yet been made in the heavens by the use of atelescope constructed upon the principle of reflexion.
At length the discoveries of Herschel introduced a new era in the historyof celestial physics. This highly-gifted individual, while occupying thehumble situation of organist of the Octagon Chapel, Bath, to which he wasappointed in the year 1766, was in the habit of devoting his leisure hoursto the study of mathematics, and various branches of physical science,more especially optics and astronomy. Happening, on one occasion, toobtain the temporary use of a 2-feet Gregorian reflector, he was sotransported with the celestial wonders which it revealed to him, that heconceived a passionate desire to procure a similar instrument for himself,and he instructed a friend in London to purchase one for him. The price
* This account is taken from the Gentleman's Magazine for 1790, Part II., p. 890.The author states, that in a trial at Westminster Hall, about the patent for making achro-matic telescopes, Mr. Hall was allowed to be the inventor; but Lord Mansfield ob-served, that “ it was not the person who locked his invention in his scrutoire thatought to profit for such invention, but he who brought it forth for the benefit of thepublic.”