354 I'HK i'uksidknt's addukss. 



Tho means now used to obtain monochromatic light arc, with 

 the exception of the lamp, much tlic same as in 183G. For 

 purposes (1) and (2) I have found nothing better than my 

 own modification of the ])rism ai)paratiis, but for (8) and 

 (4) glasses and Ihiids are more suitable, as they arc itiexpen- 

 sive and handy to use. For (5) there is nothing better than a 

 glass of a certain depth of peacock green cemented to a gliiss of a 

 certain depth of cobalt blue. The objections to the prism are 

 its cost, great size, and want of uniformity of tint throughout the 

 field, but for visual work (Iiis last objection m^ed not be con- 

 sidci'cd. Absor})tion media are now largely used, and as they 

 undoubtedly have an important future before them, a short time 

 must be devoted to tliem befoi-e we pass on. 



'i'he screens of Sir 1). Brewster were monoelu'omatic red; 

 those of Dr. Woodward were animonio-copper, blue, but not 

 monochromatic; next we have Professor Zcttnow's mixture of 

 sul[)hate of copper and bichromate of potash, giving monochro- 

 matic yellow green ; then wo have his monochromatic violet 

 screen, which is composed of two fluids, one a solution of iodine 

 in chloroform, the other being ammonio-copper. But quite 

 recently we liavi' had two new screens introduced, viz., a 

 methylen blue bv Mr. jjovibond, and a malachite green and 

 picric acid by Mr. (liiford. Neither of them is monochromatic ; 

 the methylen blue cuts out a broad band extending from the 

 u])per red to the yellow green; it passes, therefore, infra red, 

 green, blue, and violet. Photographically the infra red has no 

 clTcct, and this screen is very suilable for purpose (8); 

 visually the infra red is of no imporlance, anil as its colour is a 

 bright blue it is very pleasant to work with. The cutting out 

 of the most^ powerful of the interfering ra^^s, such as the upper 

 red, orange and yellow, renders this screen ailinlrable for })ur- 

 poses (1) and (2). 



;Mr. Gilford's green screen gives wider bands than the 

 methylen blue, for it cuts out the upper red, orange, yellow, 

 violet, and part of the blue ; it passes, therefore, infra red (a 

 trifle more than the methylen blue), green, and a little blue. 

 For photography with this screen jilates should be sensitive for 

 the portion of the spectrum near the F line. The remarks on 

 the methylen blue screen are equally applicable to this. Mr. 

 Gifford uses these screens in a highly concentrated form, so that 



