TEL 



bright objects, so that the pupil may be in its most expanded state ; it 

 requii'ing at least 20 minutes before the eye can admit a view of very de- 

 licate objects (such as faint nebulae) ; and the observation of a star, 

 though only of the 2d or 3d magnitude, disorders the eye again, so as to 

 require nearly the same time for the re-establishment of its tranquillity. 

 (Herschel, Phil. Tran.) 



(3) We should never use a greater magnifying power than we abso- 

 lutely want ; the lower the power, the more beautiful and brilliant the 

 object appears. In objects however that require great nicety to discern, 

 such as the spheroidical shape of the planets, &c. it is proper in the first 

 instance to use a considerable power, till the eye is accustomed to the 

 phenomenon, after which the power may be gradually lowered. (Hers- 

 chely Phil. Trans.) 



(I) It may be proper to observe, in order to prevent disappointment, 

 that in the prints usually given of Jupiter, Saturn, &c., the outlines and 

 all the other features of the engraving are far more distinct than we 

 can ever see them in the telescope in one view , it being the very inten- 

 tion of a copper-plate to collect together in one view all that has been 

 successfully discovered by repeated and occasional perfect glimpses, and 

 to represent it united to our conceptions. And this is the case with 

 all drawings in books of Astronomy. (Hersch. Phil. Trans.} 



(5) In attempting to determine the apparent shape or magnitude of 

 any planetary body or satellite, it is useful to compare it with some 

 other known object of a similar kind. Thus to form an idea of the pe 

 Culiar shape of Saturn, compare it with Jupiter several times in succes- 

 sion. To form some notion of the apparent magnitudes of Juno, Pallas, 

 Ceres, and Vesta, compare them with each other, or with Jupiter's sa- 

 tellites. (Hersch. Phil. Trans.) 



(6) When we wish to discover very delicate and minute objects, which, 

 with the finest instruments, are only to be seen under the most favour- 

 able circumstances, it is indispensable that we should be in a position of 

 the greatest ease ; no cramped or painful posture must distort the body 

 or irritate the mind, the whole powers of which must be concentrated 

 in the eye. (Kitchiner.) 



(7) In adjusting the telescope to close double stars, Dr. Herschel ad- 

 vises the observer previously to adjust the focus of his glass with the 

 utmost delicacy on a star known to be single, of as nearly as possible 

 the same altitude, magnitude, and colour, as the star which is to be ex- 

 amined, carefully observing whether it be round and well defined, or 

 surrounded by little Hitting appendages, as is the case when the object 

 glass is not quite perfect. (Phil Trans.) 



299 



