INTRODUCTION TO OPTICS. 

 fig. 43. 



of the sun. The microscope thus enables us to see and distinguish objects 

 which are too small to be visible to the naked eye. But there are objects 

 which, though not really small, appear so to us, from their distance. To 

 these we cannot apply the same remedy, for when a house is so far off as 

 to be seen under the same angle as a mite which is close to us, the effect 

 produced on the retina is the same : the angle it subtends is not large 

 enough for it to form a distinct image on the retina. It is impossible in 

 this case to bring the object to the eyes, but by means of a lens we 

 may bring an image of it nearer to us ; but then, the object being very 

 distant from the focus of the lens, the image would be exceedingly 

 smaller than the object itself, and in most cases it would even be so small 

 as to be invisible to the naked eye. To obviate this difficulty, we must look 

 at, the image through another lens, which, acting as a microscope, enables 

 us to bring the image close to the eye, and thus renders it visible. This 

 instrument is a telescope. In Jig. 44, the lens C D forms an image, E F. 



Fig. 44. 



A 



of the object A B ; and the lens X Y serves the purpose of magnifying that 

 image : and this is all that is required in a common refracting telescope. 

 Observe that the image is not inverted on the retina, as it usually is : the 

 object therefore appears to us inverted. When it is necessary to represent 

 the image erect, two other lenses are required ; by which means a 

 second ima^e is formed, the reverse of the first, and consequently upright. 

 These additional glasses are used to view terrestrial objects, for no incon- 

 venience arises from seeing the celestial bodies inverted. 



When a very great magnifying power is required, telescopes are con- 

 structed with concave mirrors instead of lenses. Concave mirrors pro- 

 duce by reflection an effect similar to that of convex lenses by refraction. 

 In reflecting telescopes, therefore, mirrors are used in order to bring the 

 image nearer the eye; and a lens or eye-glass, as in the refracting tele- 

 scope, to magnify the image. The advantage of the reflecting telescope 

 Js, that mirrors whose focus is six feet will magnify as much as lenses of 

 a hundred feet. 



