THE TELESCOPE. 
469 
of flint-glass C D (with, focus at E) formed to the correct 
curves., a colourless focus might he thus obtained. The result 
was the achromatic telescope. The aim of the optician was 
henceforward, therefore, to adopt the truest curves in which to 
grind and polish his glass, and to seek to unite the violet and 
red rays at the true focus F. Should the violet ray still be 
behind the red one, the telescope is under-corrected ; but should 
the flint-glass, on the contrary, err on the opposite side, the 
violet ray will have a focus beyond the red one and the convex 
glass be over-corrected. The discovery of Dollond, it may be 
imagined, created great interest at home and abroad, and was 
a great boon to astronomy, as the celestial bodies could now 
be seen equally as well, if not better, with telescopes of four 
or five feet in length than they formerly had been with tubes 
of ten times those dimensions. For upwards of half a century 
the English achromatic held the monopoly of the market, and 
it is even said, led to a profitable smuggling business ; and 
yet we believe that before the year 1828 an object-glass 
exceeding six inches in diameter had not been executed in this 
country. 
The great difficulty lay in obtaining discs of glass proper 
for the purpose. It had not only to be pure, but beyond 
suspicion, and so many tests could be applied to examine its 
quality that very few specimens escaped without the detection 
of some flaw. If a skilful optician, following the advice of Mrs. 
Glasse, could first catch that material, the subsequent opera- 
tions would give him far greater pleasure than toil, although 
immense labour and patience are required in grinding it in an 
iron basin and polishing it with mysterious powders and secret 
contrivances, of which each professor of the optic art has his 
own. The first discs of any size were those constructed by 
Guinand, who by his own unaided exertions produced speci- 
mens eight inches in diameter before the close of the last 
century, and subsequently, in conjunction with the greater 
genius of Fraunhofer, produced that miracle of skill — the 
Dorpat telescope of ten inches aperture. The successors of 
Fraunhofer have surpassed him in glasses ranging up to six- 
teen inches in diameter, which for definition and absence of 
colour are equal to the Dorpat, whilst exceeding it enormously 
in light-giving powers. What progress has been made by our 
own opticians might be seen at the last Exhibition. Various 
specimens of excellent and large foreign telescopes are to be 
met with in this country ; indeed, the three largest of Cauchoix, 
of a foot in diameter and upwards, are in Cambridge, London, 
and Sligo. 
It is not every one, however, who has an opportunity of 
possessing, or of looking through a telescope of these great 
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