A POCKET TELESCOPE 7 



add that my instrument measures when extended just over 17 inches, and 

 the object lens is i inch in diameter, or, with the mount, ij inch. It will 

 focus objects fifteen feet away, but no nearer which is exasperating sometimes, 

 until you discover the dodge by which you can focus objects as near as three 

 feet away. 



It is, then, with the expenditure of 6s. 6d. that we are going to provide our- 

 selves with telescope, microscope, camera, etc. Whatever other disbursements 

 are necessary in the making of the accessories will be chiefly a matter of pence 

 only. 



Let us begin by taking the telescope to pieces (Fig. i). It consists of four 

 tubes, three of which slide into the fourth, AB, at the end of which is the object 

 lens. This can be unscrewed, and consists of an achromatic doublet (that is, 

 two lenses, a positive and a negative, in contact with each other). This lens is 

 about 10 inches focus, and can be used, as I shall show, as a photographic lens 

 in a simple Nature camera. If dust gets between the two glasses, they can be 

 taken out of the mount for cleaning purposes by unscrewing the inner ring 

 to be found at the back of the mount. In replacing them, remember that the 

 negative, or diminishing, lens must go behind. 



The second tube of the telescope, on being unscrewed, is found to contain 



B 



C 



*~*~ ? 



< n" > 



FIG. i. 



a fixed diaphragm, with an opening of of an inch. But the third tube, CD, 

 has no such diaphragm. It is, however, an advantage to have one here, when 

 one is using a much higher eyepiece than the normal, and I have added one, 

 made of cardboard, with an aperture of of an inch in diameter, which in- 

 creases the brilliancy and definition of the object. 



There was no difficulty in making it. A little cardboard tube an inch long 

 was rolled up and glued together to fit into the end of the tube at C, and a 

 disc of cardboard, with a hole in the middle of the required size, was glued on 

 to the top of it. This diaphragm, then, is removable, being placed in position 

 only when required. 



We now come to the last tube, containing the eyepiece of the telescope, 

 and this is the most elaborate part of the instrument. There are four lenses 

 of high magnifying power, contained in two small tubes, of which one is screwed 

 into the bottom end, and the other fits loosely into the other end of the tube 

 DE, the latter being held in position by the cap at E. It will be noticed 

 that between each pair of lenses there is a small fixed diaphragm. Three 

 out of four of these lenses can be used as pocket magnifiers when required. 

 They can easily be used in the hand, held between finger and thumb ; but it 



