l86 THE ESSEX FIELD CLUB. 



mounted on a single leg, with vertical and horizontal movements. The maker, 

 James Short, was born in 1710, and originally educated for the Church. He 

 attracted the attention of Maclauren, wlio permitted him, about 1732, to make use 

 of his rooms in College Buildings for experiments in the construction of tele- 

 scopes. In Short's first telescopes the specula were of glass as suggested by 

 Gregory, but he 'afterwards used metallic specula only, and succeeded in giving 

 them true parabolic and elliptic figures. All Short's telescopes were of the 

 Gregorian form. He died in London in 1768. Short had solved the problem 

 of giving to metallic surfaces a perfect parabolic figure ; but so jealous was he of 

 his secret that he caused all his tools to be burnt before his death. ^ 



Mr. Howell spoke of the invention of the achromatic combination, generally 

 attributed to Dollond, but which he believed was really devised by Chester Hall, 

 an Essex man. 



He also mentioned that Bradley was closely connected with Gloucestershire, 

 his own native place, having been born at Sherbourne, at school at Northleach, 

 and buried at Chalford, in the wide parish of Minchinhampton, celebrated in the 

 following doggerel lines 



" Beggarly Bisley, strutting Strtnal. 



Mincinghampton and Painswick proud.' 



In reply, Mr. Crouch mentioned that he also had an old Gregorian of similar 

 construction, but larger, which had been in his family for over 150 years. The 

 definition is sharp and perfect, the metal mirrors still retaining their original high 

 polish. The instrument is mounted with horizontal rotary, and vertical movement 

 on a tripod brass stand, the length of the tubes being over 22 inches. The small 

 speculum measures J inch in diameter, and the large 3 inches, with a perforation 

 of f inch. He remarked that it was a well-established fact that the invention of 

 the achromatic lens was made by Chester Moor Hall, of New House, Sutton, 

 Essex, in 1733, who had several telescopes made, but never published his dis- 

 covery. In 1757 John Dollond, F.R.S., arrived, quite independently, at the same 

 discover^' for the correction of chromatic aberration, and put it to a practical use, 

 secured by patent. In a subsequent trial, " Dollond v. Champness," for infringe- 

 ment of this patent, Mr. Hall appears to have been quite indifferent, and took no 

 part in the action ; but on certain evidence given the priority of invention was 

 acknowledged, and accorded to him by the judge, Lord Mansfield. The earlier 

 eye-piece of Huygens, from its construction, was to a large extent achromatic. 



On the motion of the Chairman, a vote of thanks was passed to Mr. Crouch 

 for his paper. 



Mr. John Spiller, F.l.C, F.C.S., then delivered a lecture, entitled : 

 "A Few Facts about Colour-Blind.ness." 

 \_Ahsh-act.'\ 

 The structure of the eye, with its sensitive retina and " blind spot," was 

 briefly described ; likewise the nature of white light and its resolution by the 

 prism into a band of many coloured rays, known as the solar spectrum. Normal 

 vision enables us accurately to identify the various tints composing this series ; 

 but a small proportion of mankind — usually about 4 per cent, in males — is un- 

 able to appreciate nice distinctions of colour, or even to see certain well-marked 



1 For the foregoing I am indebted to Miss Gierke's " History of Astronomy " ; and to an 

 article by David Gill, our present Astronomer at the Cape of Good Hope. — W. C. H. 



