308 optics. 



and stands directly between the eye and the object 

 towards which the telescope is turned ; and will 

 hide the whole object from the eye at e, if the two 

 glasses R and S are taken out of the tube. 



Herschell chiefly makes use of a Newtonian 

 reflector, the focal distance of whose great mirror 

 is 7 feet, its aperture 6.25 inches, and powers 227 

 and 460 times ; though sometimes he uses a power 

 of 6,450 for the fixed stars; but his great telescope 

 is four feet in diameter, and 40 feet in length. 



If the metals of a Newtonian telescope are 

 worked as exquisitely as those in Herschell's seven 

 feet reflector, the highest power that such a tele- 

 scope should bear, with perfect distinctness, will 

 be given by multiplying the diameter of the great 

 speculum by 74; and the focal distance of the 

 single eye-glass may be found by dividing the focal 

 distance of the great mirror by the magnifying 

 power ; thus 6.25 multiplied by 74*, is 462, the 

 magnifying power. 



Acromatic Telescopes. — Since the construction 

 of a telescope consists in nothing more than view- 

 ing by means of a microscope or eye-glass, the 

 image which is formed in the focus of the object- 

 glass, it may seem easy to make a telescope with a 

 given object-glass, that shall magnify in any assign- 

 able degree. For if the eye-glass be rendered 

 more and more convex, the eye may be permitted 

 to approach nearer and nearer to the image ; and, 

 consequently, to view it under an angle of appa- 

 rent magnitude that shall be greater and greater, 

 as required. But this is unattainable on two se- 

 veral accounts. The first is, that spherical sur- 

 faces do not refract the rays, of light accurately to 

 a point, as has been already observed ; and the 

 second and principal reason is, that the rays of 



