gauze is almost invisible. 



new and handy Focometer. 335 



Two thin cards about the size of 

 post-cards should be taken, a hole a centimetre square should 

 be cut through both of them, and they should be gummed 

 together with the cross threads between them, the threads 

 being in the first instance long enough to project beyond the 

 cards to facilitate adjustment while the gum is wet. Waxed 

 carpet-thread, or any very stout thread with smooth edges, is 



Fig:. 3. 



WlrfW 



7— \ 



t»+WW 



7 



A supporting- pin. 



the best for giving a conspicuous and at the same time a 

 sharp image. As the cross will sometimes have to be raised 

 or lowered, the hole should be much nearer to one end of 

 the card than to the other, in order to give a greater range 

 of adjustment in mounting on the clip. One thread should be 

 vertical and the other horizontal, in order that their simul- 

 taneous focussing may serve as a check on the correct 

 orientation of the lens. 



The instrument is intended to be used by placing it on a 

 table of length not less than four times the focal distance 

 which is to be measured. A lamp is to be placed either on 

 one end of the table or on a stand opposite the end, at such 

 a height that its flame is about level with the tops of the 

 clips. The clips should be fixed as low as possible on their 

 supporting pins, unless it is necessary to raise them to suit 

 the height of the lamp. In default of a lamp at the proper 

 height, an adjustable mirror may be used instead, and made 

 to reflect a beam of light from any large gas-flame in the 

 room so that the beam shall pass along the tops of the clips. 



2 A2 





