HOW TO OBSERVE THE HEAVENS. 349 



From what has been explained, it will be apparent that the telescope, while 

 it is incapable of exhibiting to us even the nearest of the stars with any sen- 

 S'ble magnitude, may however be applied with success to obtain an approxi- 

 mate estimate of the relative distances of those stars which, by reason of their 

 r jmoteness, are invisible without its aid. By applying proper principles of 

 calculation, it is easy to determine the magnitude of the telescope which will 

 double or treble, or, in short, which will augment the range of the natural eye 

 in any required proportion. Thus, if we assume that the smallest star visible 

 t ) the naked eye, is at a distance over which light would take ten years to 

 f ass, we can find the magnitude of the lenses or reflectors which would enable 

 us barely to perceive similar stars at the distance which light would take 

 twenty years to move over; arid then, by constantly enlarging the opening of 

 I he instrument, or what is the same, by using successively telescopes of in- 

 creased powers, we may bring into view objects whose distances (supposing 

 i heir real magnitude and brightness to be the same in the main), are greater 

 and greater in known or calculable proportions. 



Sir William Herschel actually practised this method of sounding the heavens. 

 He classed the stars visible to the naked eye in twelve orders of distance, those 

 of the twelfth order (or smallest), being twelve times more distant than those 

 of the first order. A telescope which would just render visible a star twice as 

 Jistant as one of the twelfth order, and which therefore would double the range 

 of the eye, he denominated as a telescope of the second degree of space-penetra- 

 ting power. One which would bring into view stars three times more distant 

 than those of the twelfth order, he called a telescope of the third power, and 

 so on. Calculating in this way, he found that his great forty-feet telescope had 

 i! space-penetrating power of 192. To reduce this power to a still more definite 

 expression, let us call the distance of the brightest and nearest stars 1 ; that 

 of the smallest stars visible to the naked eye will then be 12 ; and that of the 

 smallest stars which could be distinctly seen with the forty-feet telescope 

 would be 192 times 12, or 2.304. 11 the distance of the nearest star be such 

 us light would take ten years to move over, the distance of the smallest stars 

 visible with this instrument would then be such as light would take 23,040 

 years to move over ! The mind is overwhelmed by the contemplation of such 

 spaces. 



The results of the application of these wonderful instruments of stellar re- 

 search in the hands of Sir William Herschel, will be stated on another occa- \ 

 sion. Meanwhile, every private observer, supplied with a moderately-good 

 astronomical telescope may, following the example thus placed before him, 

 render his labors profitable to science, by contributing to the multiplication of 

 those facts on the comparison and classification of which the extension of our 

 knowledge of the universe must depend. 



Among the objects to which the amateur can direct his attention with most 



advantage, may be mentioned the observation of periodical and double stars. 



Although there is no certain or accurate means of estimating the brightness of 



stars, still, even such approximation as an attentive observer can supply re- 



' spectingthe changes of variable stars, is not without its value. A circumstance 



( incidental to the astronomical telescope has supplied a method of determining 



[ the relative quantity of light transmitted by different stars, which is somewhat 



( more accurate than naked estimation. The instrument used for measuring 



j small angular distances is, as we have explained on another occasion, a system 



( of line wires or threads, which are fixed or moveable, according to the observa- 



J tions to be made, and which are placed in or near the focus of the object-glass 



\ \vh< re the imaue of the star is formed. The eye-glass is in fact a microscope, 



by winch this image is magnified, and by which, therefore, the threads or wires 



