82 PRACTICAL LESSONS IN SCIENCE. 



the eye-piece ; but in the case of the telescope the object is magni- 

 fied only by the eye-piece. . The object glass of the telescope serv- 

 ing as a light gatherer, an object glass of 20, 30 or 35 inches 

 collecting vastly more light than can the unaided eye. 



The Galilean telescope consists of a double convex object 

 glass and a double concave eye-piece ; the object being more than 

 twice the focal distance of the lens, the image would be inverted 

 and smaller than the object; but between the object glass and 

 its focus the concave lens is inserted which separates or disperses 

 the converging rays, and the image is seen erect and enlarged. 

 The ordinary opera glass consists of two Galilean telescopes. 



The terrestrial telescope used in surveying frequently consists 

 of four double convex lenses; the inverted image of the ordinary 

 telescope being confusing, two more lenses are added so as to give 

 an erect image of the object. 



In the reflecting telescope light is gathered by a large concave 

 mirror, and the image formed is enlarged by a double convex 

 lens. 



The magic lantern is an instrument by which the image of a 

 small object greatly magnified may be thrown upon a screen. It 

 consists of a dark box in which there is a strong light ; behind 

 this light there is a concave mirror, and in front of it an opening 

 from which projects a tube. At the entrance of the tube is 

 placed a double convex lens. or two piano convex lenses; inside 

 of this tube slides a smaller tube in which is fixed another 

 double convex lens. The picture or object is placed between the 

 lenses. 



The first lens receiving light from the lamp, and by reflection 

 from the mirror, concentrates it upon the transparent object or 

 picture, strongly illuminating it. Light from the object goes on 

 through the second lens to a distant screen and there forms a 

 large and perfect image. 



The camera consists of a dark box divided by a partition or 

 screen of ground glass. The box has openings in two opposite 

 sides, from one of which projects a tube in which slides a smaller 

 tube carrying a double convex lens. If the lens is properly 



