278 Dr Andrew Fyfe on the Comparative Vahie of 



that transmitted When, however, it is so situated that 

 the transmission from behind, and the reflexion from be- 

 fore, are of the same intensity, then the spot is invisible ; 

 the paper appears as if uniform throughout Now, with 

 a light of uniform intensity placed behind, the transmis- 

 sion will always be the same. If another light before the 

 paper requires to be at the distance of 5 inches, and another 

 at 10 inches, to cause, one after the other, the spot to disap- 

 pear, then, according to the usual law, they are giving light 

 as 25 and 100, i. e., 1 to 4. After a little practice I have 

 found this photometric process extremely delicate. It has 

 many advantages over the shadow test : for instance, the 

 difference in the colour of the shadow is avoided ; besides, the 

 trials can be conducted without darkening the room, unless 

 there are cross lights or sunshine directly into the apart- 

 ment. 



With regard to the light to be placed at the back of 

 the screen, the only uniform source with which I am ac- 

 quainted is a wax or spermaceti candle, of the same dia- 

 meter, and with the same thickness of wick. With regard 

 to the modes of preparing the paper, I at first used sperma- 

 ceti melted, as recommended by Mr King, and applied it to 

 bibulous paper in various ways ; but I never succeeded in 

 getting it uniformly spread over the surface, — it was gene- 

 rally thicker at one part than at another, which gave rise to 

 a difficulty in fixing the distance at which the light should be 

 placed. After trying different niethods, I have, however, suc- 

 ceeded in getting the paper properly prepared. The process 

 I now follow, is to dissolve spermaceti in distilled oil of 

 naphtha, till it gives a mixture, which at natural tempera- 

 tures is solid ; but is liquefied by the application of a very 

 slight heat, such as by holding the vessel in the hand for 

 some time. Wlien fluid, it is to be applied by a hair-pencil 

 to the paper, leaving a part of about the size of a half-crown 

 piece at the centre uncovered. After this the paper is held 

 horizontally over a lamp, and very cautiously heated, by which 

 all inequalities disappear. I prefer the fine cream-coloured 

 paper now much used as letter paper. 



