j li; Eisen: A successful achromatic light-tilter. XIV. 4. 



position it wonld be difticnlt to remove. It shonld slide into the 

 adapter, not be screwed into it. 



The first thing in preparing the cell is to dissolve the original 

 eement holding the glass plates together and to recement them with 

 some eement not solnble in aleohol. If this eement contains ariy 

 aeid , it shonld be carefnlly eliminated after the cell has properly 

 dried. The glass cell which I use is 37 mm in diameter and 

 7 mm deep, — both being outside measnrements. The circular rim of 

 the cell contains two small holes through which the cell may be filled 

 or emptied, a common pipette dropper with nibber bnlb being nsed. 



The liquid is a mixture of two separate Solutions in absolute, 

 nncolored aleohol, of apparently equal density of color. One Solution 

 contains cyanin (Grübler) , the other methylen - blue „0" patent 

 (Berlin Actiengesellsch. f. Anilinf.). The Solutions shonld be weak, 

 so that when held up in a dropper against the window the color 

 is that of a deep blue sky. 200 parts of the cyanin Solution with 

 one part of the methylen-blue shonld be mixed in a bottle and kept 

 as stock Solution. This mixture shonld be flltered. It will keep 

 about one month. 



When the cell is filled, it shonld be elosed with a rubber band, 

 this being the easiest niethod of closing, thongh it does not entirely 

 prevent evaporation. The density of the Solution shonld be just 

 sufficient to eliminate all the yellow rays, leaving no trace in the 

 field of either blue or yellow. The light shonld be creamy white. 

 It will require, perhaps, a few hours experiment to find the proper 

 strength, density, and color of the Solution for the particular light- 

 source nsed ; but when this has once been found, it reqüires only a 

 verv few minutes to replenish the cell. If the field is bluish, it is 

 a sign that there is too much methylen-blue in the mixture ; if too 

 red, there is too much cyanin. The proportions of these two stains 

 varies aeeording to the strength of the light used. Too much blue 

 is to be avoided , as such light would be tiresome to the eyes — 

 what I shonld call a dead light. When the proper proportions have 

 been ascertained, which, for my light source, 1 find to be I of the 

 blue to 200 of the cyanin, the result is a remarkably soft, white 

 light, very soothing to the eyes, instead of being injurious to them. 



When, as before stated, the liquid begins to change through 

 exposure, and the light passing through the filter becomes bluish, 

 a drop of the cyanin Solution may be added , or the liquid may 

 be replaced by fresh. 



