TEL 

 their value tvherx * ~ 0, and we shall have by Maclaurin'a theorem 



In like manner, 



TELESCOPE, theory of.(Coddington t Wood.) 



1. Astronomical Telescope. 



Let F, F' represent the focal lengths of the object and eyo glass j then 



F 

 the magnifying power is ,. 



Cor. The linear magnitude of the greatest visible area is measured by 

 the /, which the diameter of the eye glass subtends at the centre of the 

 object glass, increased by the difference between the /'s which the dia- 

 meter of the object glass subtends at the image, and at the eye glass. 



2. Galileo's telescope. 



The magnifying power as before .= . 



Cor. The linear magnitude of the field of view, when the eye is placed 

 close to the concave lens, is measured by the angle which the diameter 

 of the pupil subtends at the centre of the eye glass, increased by the dif- 

 ference between the /'s which the diameter of the object glass subtends 

 at the pupil, and at the image. 



3. HersclteVs and Newton's telescope. 



L3t/and F' be the focal lengths of the speculum and eye glass ; then 

 the magnifying power = ^ 



Cor. The field of view is nearly equal to the apparent magnitude of 

 the eye glass seen from the speculum. 



4. The Gregorian and Catsfgr&in't telescope. 



Let/,/', F, be the focal lengths of the great and small mirror, and the 

 lens respectively, I the distance of the mirrors; then the magnifying 

 power of the Gregorian is nearly 



(*-/') 

 fV ' 

 and of Cassegrain's la 



294 



