54 TRANSACTIONS AND PROCEEDINGS OF THE [Sess. lv. 



There was nothing new about the apparatus except this, that 

 he had put on the front of it an iris diaphragm of the 

 newest design, which he believed to be the first time it was 

 used in this way. The effect it had was this, that by a 

 simple turn of the lever handle the size of the disc on the 

 screen could be reduced from full aperture to any extent, 

 and in this way any portion of an object on the screen could 

 be brought more prominently into view when the image was 

 restricted to that particular part. The iris diaphragm was 

 no new invention. The first who used it, so far as he was 

 aware, was the optician Eamsden, of London, about the end 

 of last century. He employed a couple of brass plates with 

 rectangular apertures cut in each. These plates were made 

 to approach and recede from each other by mechanical 

 means, so that the apertures in each met and gradually 

 closed. The opening, however, was not a round one. 

 Eamsden applied this to the telescope only, and the use of it 

 does not appear to have been brought prominently into 

 notice until Mr Wales, an American optician, adapted a 

 diaphragm of "this kind to his microscopes about the 

 time of the Centennial Exhibition in America. He, 

 however, constructed his iris in a different way. He had a 

 short tube, the upper part of which was shaped like a cone 

 inside. Into this conical portion a piece of thin tube, split 

 into a number of leaves, was made to screw up and down by 

 a simple arrangement. The split leaves were thus compelled 

 to overlap each other and close the aperture. This pro- 

 duced a very nearly round hole, varying in size according to 

 the degree to which the overlap took place. This form is 

 still made and used in the cheaper form of microscopes, such 

 as the " Star " microscope, by Messrs Beck & Beck of 

 London. 



The iris now shown, however, is made in a different way. 

 Thin steel plates, about 1-100 of an inch thick, and very like 

 the shape of one's bent finger, and varying in number from 

 10 to 15, are fixed at one end by a pin through each, and on 

 which they are all made to turn close together by means 

 of pieces attached to the other end of each plate, working 

 in a .second slotted circular frame. This form of iris was 

 first made in a ruder way by means of four plates only, but 

 these merely gave a near approach to a circle. That now 



