Work for the Telescope. 269 



wliom we amateurs are so deeply indebted, will teach us that 

 " many things deemed invisible to secondary instruments are 

 plain enough to one who ' knows how to see them •/.... the 

 eye itself improves, and the vision becomes sharper under prac- 

 tice, insomuch that the telescope seems to improve." 



Assuming, then, that the student is anxious to make the best 

 of his means, whatever they may be, we shall previously beg his 

 attention to a few remarks which, in some cases at least, may be 

 found of service. 



No observer who hopes to see anything to advantage will 

 think of stationing himself at a window, unless indeed his tube 

 is long enough to carry the object-glass well out into the open 

 air. Excepting at those seasons of the year when the external 

 and internal temperature are equalized, the intermixture of 

 warmer and cooler currents at an open window destroys all hope 

 of distinctness with any magnifying power worthy of the name. 

 It seems to be from this very cause on a large scale that the 

 undulations and flickerings arise of which astronomers com- 

 plain so much and so often. The success of Professor Piazzi 

 Smyth's " Experiment" on the Peak of Teneriffe demonstrates 

 how much the definition of our telescopes would be improved, 

 could we leave the less equable regions of the atmosphere 

 beneath our feet ; but at any rate we need not artificially aggra- 

 vate the evil which at our ordinary level we cannot avoid. The 

 indistinctness thus produced is found to grow rapidly with 

 increase of aperture, as indeed must be the natural consequence 

 of including a greater amount of agitated medium in the cylinder 

 of rays which the telescope brings to a focus ; and hence the 

 Roman astronomers who, even in that serene and pellucid air, 

 find many a cause of complaint, occasionally contract the 9^-inch 

 aperture of their noble achromatic to little more than five inches, 

 in order to diminish the disturbance in less favourable nights. 

 So far the possessors of smaller instruments are not without 

 their advantage : they do not see so much, but they see more 

 quietly, and they can employ a greater number of nights with 

 tolerable comfort. The unsteadiness of a boarded floor is an- 

 other argument for out-door observation. We are, generally 

 speaking, scarcely aware how extensively vibrations may be 

 propagated, even through materials of little apparent elasticity ; 

 but the shaking of the earth for many miles round an exploding 

 powder-mill — fifty or sixty miles in the terrible Hounslow ex- 

 plosion in 1850 — is a convincing instance of this ; and when 

 the celebrated optician Troughton used a small observatory in 

 Fleet Street, during the two quiet hours — from 2h. till 4h. — in 

 a London night, he used to perceive the approach of a distant 

 carriage by the vibrations of a star in the telescope some con- 

 siderable time before the slightest sound was audible. Hence, 



