new and handy Focometer. 337 



frame is just sufficient to keep them from working while the 

 instrument is being carefully set down. The chief difficulty 

 is from flexure. 



Instead of receiving the image on a screen, it can be viewed 

 in mid air. For this purpose I mount the cross on one of 

 the two end clips, and a piece of wire gauze about the size of 

 the palm of my hand on the other, setting the wires at a 

 slope of 45° by way of contrast with the upright cross. The 

 end which carries the cross should be turned towards the 

 strongest light ; as this renders the cross more visible to an 

 observer behind the gauze, and also renders the glistening 

 wires of the gauze more visible when the observer stations 

 himself behind the cross. The adjustment for focus is made 

 by lengthening or shortening the frame till parallax is re- 

 moved. This is a very convenient way of establishing 

 experimentally the fact of the interchangeableness of object 

 and image. 



The instrument can also be employed to illustrate the 

 general law of variation of conjugate focal distances, the lens 

 being for this purpose shifted from the central pin to any one 

 of the other pins, and the frame being then extended till the 

 image is correctly focussed. Regarded as an optical bench, 

 the instrument is remarkably light and handy. Its weight, 

 including screen, cross, wire-gauze, and lens, is 21b. 10 oz. ; 

 and a lecturer can carry it through the streets of a town 

 without inconvenience. 



The dimensions and number of bars of the instrument as 

 exhibited are recommended as the most convenient for general 

 purposes. Ten bars only were constructed for the first trials, 

 and any number included in the formula hi + 2 might theo- 

 retically be employed. 



In order to prevent looseness at the joints, it would be well 

 to make the holes in the bars bear against a cone below and 

 another cone above, with a very slightly tapering wedge for 

 adjustment, as indicated in fig. 3. 



If the instrument were to be set up permanently in one 

 place, guides might be used for compelling the middle row of 

 pins to travel without rotation, or the pin on which the lens 

 is mounted might be a fixture ; but as long as portability is 

 to be preserved, 1 do not think that any arrangements for 

 automatically preventing rotation would be practically bene- 

 ficial. It is only in the large movements which precede the 

 final adjustment that rotation occurs to any injurious extent. 



The instrument has been constructed from my drawings by 

 Messrs. Yeates of Dublin, and the cost is trifling. 





