



Different Kinds of Coal for the purpose of Illumination. 165 



Before concluding I wish to advert briefly to a mode of fixing 

 the illuminating power, which, so far as I am aware, has not been 

 publicly noticed in any of the papers lately printed. Dr. Lyon 

 Playfair, who described it to Mr. King of the Gas Works, Liver- 

 pool, from whom I first got the account of it, ascribes it to Profess- 

 or Bunsen of Marburg ; and, accordingly, as I understand, he has 

 the merit of proposing it. It consists of a sheet of paper, besmear- 

 ed with spermaceti, excepting at a small part at the centre, by 

 which the besmeared portion becomes more pervious than the 

 other to light ; and consequently a light placed behind it, causes 

 a dark spot on that part not covered. When another light is placed 

 before the paper, the spot is distinctly visible, if that light be pla- 

 ced at such a distance as to cause the reflexion from the paper to be 

 either of greater or of less intensity than that transmitted. When 

 however, it is so situated that the transmission from behind, and 

 the reflexion from before, 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 transmission 

 will always be the same. If another light before the paper re- 

 quires to be at the distance of 5 inches, and another at 10 inches, 

 to cause, one after the other, the spot to disappear, then, accord- 

 ing to the usual law, they are giving light as 25 and 100, •'. e., 1 

 t0 4. After a little practice I have found this photometric pro- 

 cess extremely delicate. It has many advantages over the shad- 

 ow test : for instance, the difference in the color of the shadow 

 is avoided ; besides, the trials can be conducted without darken- 

 ing the room, unless there are cross lights or sunshine directly 

 into the apartment. 



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

 the only uniform source with which I am acquainted is a wax or 

 spermaceti candle, of the same diameter, and with the same thick- 

 ness of wick. With regard to the modes of preparing the paper, I 

 at first used spermaceti melted, as recommended by Mr. King, and 

 applied it to bibulous paper in various ways ; but I never succeed- 

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

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

 ficulty in fixing the distance at which the light should be placed. 

 After trying different methods, I have, however, succeeded in 

 getting the paper properly prepared. The process I now follow, 

 18 to dissolve spermaceti in distilled oil of naphtha, till it gives a 

 fixture, which at natural temperatures is solid; but is liquefied 

 b y the application of a very slight heat, such as by holding the 

 vessel in the hand for some time. When lluid, 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-colored 

 paper now much used as letter paper. 



