CHAP, iv.] THE TELESCOPE. 263 



using two convergent lenses of the same focus, o"o"', placed between 

 the system o' of the astronomical eye-piece and the real image of the 

 object-glass, a, I, the virtual image is made erect, as it is easy to 

 see by following the path of the rays in Fig. 201. We see there- 

 fore that the eye-piece system of the erecting or terrestrial telescope 

 is formed of three or four lenses. 



The advantage of this combination is that the images are erect, 

 which for terrestrial objects is necessary. The inconvenience lies in 

 the feebleness of the light. The light absorbed and reflected by the 

 passage through two extra lenses is the cause of this. 



In the present day such telescopes are made of all dimensions 

 and very varied powers, both for useful applications as well as for 

 amusement. Before the invention of the electric telegraph, those 

 who worked the aerial telegraphs used telescopes to see the signals 

 clearly, with apertures of 8 or 9 centimetres and 2'50m. focal dis- 

 tance. Sailors use similar instruments but of smaller dimensions, on 

 account of being more convenient to handle on board ship. Night 

 glasses, of which they make frequent use, are either telescopes with 

 a simple eye-piece like astronomical refractors, or with an object- 

 glass of large diameter in order to give the greatest possible light and 

 to allow of observation when the light is dim. For houses in the 

 country more powerful glasses are constructed, as they can be fixed on 

 stands of various forms ; they are furnished with a number of eye- 

 pieces, some terrestrial and others astronomical, of different magnify- 

 ing powers, and with these astronomical amateurs can make many 

 interesting observations. 



IV. KEFLECTING TELESCOPES. 



A reflector, or catadioptric telescope, differs from a refractor in this 

 way ; the object-glass is replaced by a concave mirror, which gives 

 a real image of the object, situated at its principal focus when the 

 object is at an infinite distance. By adjusting the eye-piece properly 

 for the examination of this image, the magnification wished for can 

 be obtained as in the refractor. The substitution of a mirror for the 

 lens was suggested by Zucchi in 1616. But Gregory, an English 

 astronomer, deserves the credit of the first effective application, and, 



