342 PHILOSOPHICAL TRANSACTIONS. [aNNO 1753. 



arises from 1 very different causes ; that is, from different degrees of refrangi- 

 bility of light, and from the figure of the sphere, which is not of a proper curva- 

 ture for collecting the rays in a single point. The object-glass is chiefly aftected 

 by the first of these; nor has there been yet any method discovered for rectifying 

 that aberration so, as in the least to remove the indistinctness of the image 

 arising from it. We are therefore reduced to the necessity of contracting their 

 apertures, which renders it impossible to magnify much, without very long glasses. 



of science encouraged his pursuits. Under such favourable circumstances Mr. D. engaged in the dis- 

 cussion of a subject which then interested the public philosophical mind of all Europe. Sir Isaac 

 Newton had declared, in his treatise on Optics, p. 11 2i " That all refracting substances diverged the 

 prismatic colours in a constant proportion to their mean refraction ;" and drew this conclusion, " that 

 refraction could not be produced without colour;" and consequently " that no improvement could be 

 expected in the refracting telescope." No one doubted the accuracy with which Sir Isaac had made the 

 experiments; yet M. Euler and some others were of opinion that his conclusion went too far, and 

 they maintained that in very small angles refraction might be obtained without colour. Mr. D. how- 

 ever was not of their opinion, but defended Newton's doctrine with much ingenuity and learning ; as 

 appears by the letters that passed between Euler and Dollond on that occasion, and published in this 

 same 48th vol. of the Philos. Trans, p 287 ; where Mr. D. contended that, " if the result of the 

 experiment was as described by Newton, there could not be refraction without colour." 



Mr. D.'s active and accurate mind however could not rest satisfied barely with an experiment made 

 by another, which he could perform himself Accordingly he began the examination himself in the 

 year 1757, which lie assiduously prosecuted till June 1758, when he found the result to be very dif- 

 ferent from what he expected, and from what Sir Isaac had related. He discovered " the difference 

 in the dispersion of the colours of light, uhen the mean rays are equally refracted by different mediums." 

 The discovery was complete, and he immediately drew from it this practical conclusion, "' That the 

 object-glasses of refracting telescopes were capable of being made without being aifected by the dif- 

 ferent refrangibility of the rays of light." His account of the experiments was printed in the Philos. 

 Trans, vol. 50, p. 743, and the same year he was presented by the r. s. with the annual gold medal 

 as a reward of his discoveries, though he had not yet become a member of the Society. This disco- 

 very however no way affected the points in dispute between Euler and Dollond respecting the doctrine 

 advanced by Newton. A new principle was thus discovered, which had no part in the former reason- 

 ings, and it was reserved for the accuracy of Mr. D. to have the honour of making a discovery 

 which had eluded the observation of the immortal Newton. This new principle being now esta- 

 blished, Mr. D. was soon able to construct object-glasses correcting the different refrangibility of the 

 rays of light, and the name achromatic was given to them by Dr. Bevis, though it seems that M. La- 

 lande had said he conferred that name. Mr. D.'s improvement in refracting telescopes has been of 

 great advantage in astronomy, having been applied to fixed instruments ; by w hich the motions of 

 the heavenly bodies are determined to a much greater exactness than by means of the old telescopes. 

 Navigation has also been much bt- nefited by applying achromatic telescopes to the Hadley's sextant. 



In the beginning of the year 1761 Mr. D. was elected f. r. s., and appointed optician to the king. 

 But he did not long enjoy these honours : for, on the 30th of November, the same year, a fit of 

 apoplexy in a few hours terminated his life, at 55 years of age. Besides Mr. Peter Dollond, above 

 mentioned, our author's family, at his death, consisted of 3 daughters, and another son, Johu, who 

 with his elder brother Peter, carried on the optician's business with the greatest reputation to the time 

 of the death of the younger brother John, which happened in the year 1805. And the business is still 

 carried on by the elder brother Peter, in conjunction with his nephew Mr. Huggius Dollond. 



A larger account of the Dollonds may be seen in the Philos. Magazine, vol, xviii. p. 47, from 

 which the above has been abstracted. 



