Page:Gods Glory in the Heavens.djvu/292

2S8 inequality in its constitution produces distortion, or indistinctness in the image. If the glass is not perfectly homogeneous, the refraction is disturbed; but in the case of the reflecting telescope, the reflection is the same, however much the surface of the speculum may vary in density. This results from the law, that the angle of reflection is the same, whatever be the nature of the reflecting substance; while, in the case of refraction, the angle altogether depends on the nature of the refracting body. It is obvious, therefore, that if the lens be not homogeneous throughout, it will not possess that equality of action which perfect definition requires. But it is exceedingly difficult to obtain glass of large dimensions free from inequalities. It may be perfectly good to a cursory inspection, while, submitted to proper tests, it is wholly worthless for astronomical purposes. From this source of difficulty the reflecting telescope is entirely free, and, consequently, far greater dimensions may be attained.

The reflecting telescope, however, labours under the great disadvantage of being very cumbrous, as compared with a refracting telescope of the same power. A small refracting telescope, worked comfortably under the shelter of a dome, with nice adjustment and easy movements, usually performs more satisfactorily than reflectors, with lofty and unwieldy scaffolding, standing out of doors and the observers to the severities of the