TEL 



Proper size of telescopes. 



The smallest achromatic that can be used with success for astronomi- 

 cal purposes is the 3J feet, aperture 2% inches. (Kitchiner.) 



Magnifying powers of telescopes. 



For day purposes, a power of CO or 100 is the maximum that can bo 

 generally used in this country, except on very fine days, and on objects 

 uncommonly well lighted up. In telescopes of different apertures, the 

 maximum power for day purposes is had by multiplying 1 the diameter of 

 the object speculum or glass in inches by 30. For astronomical pur- 

 poses thb rotatory motion of the earth prevents the application of a much 

 higher power than 300 being used with any advantage : when a higher 

 power than 300 is used, it requires uncommon dexterity both to find the 

 object and manage the instrument. The following powers are proper 

 for a fine achromatic. (1) A comet eye piece, made with t\vo piano con- 

 vexes not magnifying more than 12 or 15 times, which is also a delight- 

 ful eye piece for viewing nebula? and the milky way. (2) For a series of 

 powers for planetary observations, multiply the diameter of the object 

 glass in inches by SO, 30, 40, 50 and CO ; this last is the maximum that can 

 be used for the planets, and requires a very perfect telescope, and every 

 circumstance to be favourable, to admit of its application with good ef- 

 fect. (3) A positive eye piece magnifying SOO times for close double 

 stars; yet unless the telescope be an uncommonly fine one, a higher 

 power than 200 only renders the object less distinct. (1) A circle of six- 

 single double convex lenses magnifying 50, 100, 150, 00, 300, "and 4^0 

 times, but when the highest power is used, the distinct field of view ia 

 reduced to a very small diameter. (Kitchiner.) 



Eye glasses for telescopes. 



In very delicate observations Herschel observes, no double eye glass 

 should be used, as that occasions a too great waste of light. With the 

 double eyeglass he could not see the belts of Saturn, which he very plain- 

 ly saw with the single one. Of single glasses he decidedly prefers con- 

 cave to convex glasses, as they give a much more distinct ircnge. Their 

 very small field of view is a considerable imperfection, but in objects 

 sucli as double stars, or the satellites of Saturn, and the Georgian, this 

 inconvenience is not so material. (Phil. Trans.) 



Best criterion of a good telescope. 



The most difficult object to define in the day time, and the best test of 



the distinctness and correctness of our instruments, is the dial plate of a 



watch, when the sun shines upon it, placed about 100 feet from the glass. 



la the night time a fixed star of the first magnitude is the best test, aa 



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