a M. Fraunhotei" on the Construction 



Tlie power of a telescope may be best seen by a comparison 

 with another directed on the same object. The great impedi- 

 ments in observing with large telescopes are the imperfection 

 of the air, and especially an apparent undulation. These im- 

 pediments are increased with large instruments, in proportion 

 to the squares of the diameter of the object-glasses, but the 

 effect increases only in proportion to tlie diameter ; whence, 

 although the sky may appear clear, and the air in that respect 

 be but slightly imperfect, no observations can be made with 

 large instruments. As the air is perfect in this respect but 

 few days in the year, we chose for the purpose of ascertain- 

 ing the proportionate effect of the large telescope, a terrestrial 

 object, fixed for this purpose ; as by this means the space of 

 air which we had to look through being smaller, its imperfec- 

 tion would be less injurious. The trials made in tliis manner 

 have shown that the effect of the great refractor increases, as 

 it should, in proportion to the magnitude of the diameter of 

 the object-glasses. It would lead us too far, were we to enu- 

 merate all tlie means which were employed, for instance, only 

 for bringing the axes of the glasses perfectly into one line, to 

 counteract the contraction and expansion of the metal rims of 

 the object-glasses in different temperatures, &c. ; circumstances 

 which we had to attend to, in order to secure the greatest 

 effect of the instrument. 



One of the greatest impediments found hitherto in the ob- 

 servation of celestial objects, by means of large telescopes, is 

 the apparent diurnal motion of the stars, which increases in 

 proportion to the size of the instrument; so that the stars 

 lying towards the equator remain but a very short time within 

 the field of view of a strongly magnifying telescope, and tra- 

 vei-se it very j-apiilly. However small the motion that may be 

 given to the instrument by means of screws, and for the pur- 

 pose of following them, it will receive oscillations which will 

 be larger in proportion to the size of the telescope. Before 

 the instrument has come to rest, the star will have crossed the 

 field of view, so that the observer will see it perhaps only for a 

 few moments, and as it were by accident, under favourable cir- 

 cumstances ; circumstances which will be the more rai-e, as a 

 star is seen to the greatest advantage only in the centre of the 

 field. These difficulties could only be removed if the tele- 

 scope could be made to follow the stars without the interven- 

 tion of a human hand, whether their motion be apparently 

 slow, as at the }i()le, or rapid, as at the. equator. 



For this reason the telescope has been mounted in a pecu- 

 liar inanner on a parallactic principle ; i. e. one of the two 

 principal axes, on which it is made to turn, is so inclined to- 

 wards 



