HEESCHEL. 207 



without injustice neglect to remark that a front-view telescope was 

 already described in 1732, in Volume VI of the collection entitled "Ma- 

 chines and Inventions approved by the Academy of Sciences." The author 

 of this innovation is Jacques Lemaire, who has been unduly confounded 

 with the English Jesuit, Christopher Maire, assistant to Boscovitch, in 

 measuring the meridian comprised between Rome and Rimini. Jacques 

 Lemaire, having only telescopes of moderate dimensions in view, was 

 obliged, in order not to sacrifice any of the light, to place the great mir- 

 ror so obliquely that the image formed by its surftice should fall entirely 

 outside the tube of the instrument. So great a degree of inclination 

 would certainly distort the image. The front-view construction is admis- 

 sible only in very large telescopes. 



I find in the Transactions for 1803 that, in solar observations, Herschel 

 sometimes employed telescopes, the great mirror of which was made of 

 glass. It was with a telescope of this kind, seven feet long, and six inches 

 and jthree-tenths in diameter, that he observed the transit of Mercury 

 on the 9th of ISTovember, 1802. 



Practical astronomers know how much the mounting of a telescope 

 contributes to produce correct observations. The difficulty of a solid 

 yet very movable mounting increases rapidly with the dimensions and 

 weight of an instrument. We may then conceive that Herschel had to 

 surmount many obstacles in mounting a telescope suitably of which the 

 mirror alone weighed ujiward of 1,000 kilograms, (a ton.) But he 

 solved this problem to his entire satisfaction by the aid of a combination 

 of spars, pulleys, and ropes, of which a correct idea may be formed 

 by referring to the wood-cut given in Arago's Treatise on Popular 

 Astronomy, (Vol. I.) This apparatus, and the different stands that 

 Herschel devised for telescopes of smaller dimensions, assign to that 

 llustrious observer a distinguished place among the most ingenious 

 mechanics of our age. 



The public in general — I may even say the greater j)art of astronomers 

 — know not what was the eflect that the great forty-foot telescope had in 

 the labors and discoveries of Herschel. Still, we are not less mistaken 

 when we suppose that the observer of Slough always used this telescope 

 than in imagining, with Baron von Zach, (see MonatUche Correspondenz^ 

 January, 1802) that the colossal instrument was of no use at all; that it 

 did not contribute to any new discovery f that it must be considered as a 

 mere object of curiosity. These assertions are distinctly contradicted 

 by HerschePs own words. In the volume of Philosophical Transactions 

 for the year 1795 (p. 350) I read, for example: "On the 28th of August, 

 1789, having directed my telescope (of forty feet) to the heavens, I dis- 

 covered the sixth satellite of Saturn, and I perceived the spots on that 

 planet better than I had been able to do before." (See also relative to 

 this sixth satellite the Philosophical Transactions for 1790, p. 10.) In 

 that same volume of 1790 (p. 11) I find: "The great light of my forty- 

 foot telescope was so useful that on the 17th of Sei)tember, 1789, I re- 



