On the Size lest adapted for Achromatic Glasses. 127 



of telescopes, or microscopes, is measured by the proportion of 

 the diameter of the original pencil to that of the pencil which 

 enters the eye. 



The degree in which magnifying power may be advantageously 

 applied, depends so much on the perfection of the telescope and 

 the state of the atmosphere, that it is hardly possible, by any 

 general rules, to fix precise limits to it : but, to afford an oppor- 

 tunity of trying this and many other entertaining experiments, 

 the day eye-piece should have a pipe-drawer ; and the screw, 

 which receives the tube containing the two first glasses, should 

 be the same as the screw which fixes the eye-drawer to the tele- 

 scope : and the two first eye-glasses should be made to separate 

 (by a sliding tube within the pipe-drawer) from the third an^ 

 fourth. This will give a very pleasing variety, and be extremely 

 convenient \fo those who wish to obtain a certain, exact degree o* 

 magnifying power. 



For large adjustments, and also that the telescope may be 

 used for near objects, and occasionally be made to supply the 

 place of a microscope, it should have a sliding tail-piece ; and 

 the tooth and pinion for the fine adjustment should be made 

 carefully, so as to mo%'e easily and smoothly, that it may not shake 

 the glass while adjusting it. This is one of those defects we 

 must expect to find in instruments which are so very rarely used 

 bv those who make them — the workman not being aware of tlie 

 great importance of the telescope being perfectly steady during the 

 adjustment of the focus. For this purpose, there should be two 

 steadying sliding tubes applied from the eye-end of the telescope 

 to the stand. These will in great measure prevent the vibra- 

 tions, which are such impediments to vision. When the eve is 

 perfectly satisfied with the adjustment of the focus, let the tele- 

 scope be so placed that the object may pass through the field, 

 the instrument remaining at rest during the time: this answers 

 better than running after it with rack-work. 



The telescope should be suspended in the ceatre of gravity, 

 and mounted on a portable and folding mahogany stand, with 

 divided circles, and an utiiversal polar adjustment. If the in- 

 strument be then placed in the plane of the ecjuator, only one 

 motion will lie re(iuired to follow the object; which, when large 

 magnifiers are used, is a very great advantage, as the tremors 

 occasioned by the movement of the rack-work are of course pro- 

 .portionably diminished. And be it always remembered, that 

 steadiness is of the first importance. When high magnifiers are 

 used, we need every assistance that can be contrived ; as, even 

 with the best constructed stands, a person walking in the room 

 will prevent our seeing distinctly; nay, t!ie very pulsation in the 



body 



