Letheby's and Evans's photometers are sirnilar in construction, and both depend upon the prin ciple of Bunsen's. Letheby's consists essentially of a long bar, at each eud of which are supports for a light, one being the standard candle upon a Keates's candle-balance. Upon the rod, slides a box, with holes on each side and in front ; it contains the semi-transparent paper with the opaque spot. The box is moved until the spot disappears, when a pointer attached to the box indicates on a scale the intensity of the unknown light in terms of the standard. Evans's is a similar instrument, but the box is fixed, and the lights rnove along the bar.
In gas-testing, one or other of these instrurnents is usually employed, with a great number of adjuncts, such as gas-meters, pressure-gauges, &c., &c. The gas is burnt at the rate of 5 cub. ft. an hour ist a No. 1 Sugg's London Argand for 14- to 16-candle gas, and in a Sugg's No. 7 steatite bat's-wing, for cannel gas. Many precautions have to be taken to correct the meter as to the rate of burning. Observations are taken every minute for ten minutes, and an average of the whole is taken as the result. A " jet photometer " is used as tt, rough and ready test in gas-works ; it depends on the fact that, to miintain a flame or jet at a constant height (from a given circular orifice), the poorer the gas in quality, the greater is the pressure nf gas required. This pressure can be delicately measured, and, with the aid of tables, can be translated into illuminating power in candles.
Sugg's new " patent illuminating-power meter " depends upon an extension of this principle.
The Dispersion Photometer.When very intense lights, such as the oxy-hydrogen, the magne sium, or the electric, have to be compared with gas or candles, it would be very inconvenient to remove the stronger light to the necessary distance (50 ft, or mare) from the screen. This difficulty has been ingeniously overcome by passing the intense light through a concave lens, thus dispersing it, and lessening its intensity. The curvature and focal length of the lens being known, the amount of dispersion is easily calculated, aud this dispersed light, with its intensity thus diminished to a known extent, is ernployed in the photometer. In the case of the electric light, it is usual to naake two observations, one through green, the other through red glass.
Jansen has just constructed a photographic photometer, consisting of a frame with a sensitized plate, before which, and in the path of the light-rays to be measured, a screen with triangular perforations is made to pass. A gradation of shade, decreasing from the base of the triangles towards the apex, is thus obtained, and points of equal ,hade indicate equal intensity. It is stated that he has in this way been able to express the illuminating power of some of the stars in terms of that of the sun, aud it is expected that he will be able to construct a definite solar scale, to which all artificial lights may be referred, The recent wonderful researches of Alex. Graham Bell, assisted by Tainter, upou sounds pro dnced when beams of light, interrupted rapidly by perforated discs revolving at high speed, fall upon various substances, seem to paint to the possibility of constructing a,n instrument in which the different intensities of two lights will make themselves evident in differences of audible tones, when the rays from each of them fall upon suitably constructed receivers containing lamp-black, and provided with hearing tubes to convey to the ear the sounds produced by the sueceasive impact of interrupted light-rays.
ll/timinating Value.In order to arrive at a true eatimate of the actuel money-value of any illuminating material, it is neceaaary to take into account not merely ita light-intensity, as deter mined by tbe photometer, but also the rate at which it burns, and ita price (per lb.). There are therefore three variable elemente, each oae of which must be duly considered. If, for example, paraffin aud stearine (or composite) candles give equal light photometrically, and 1 lb. of stearine candles lasts 48 houra, while 1 lb. of paraffin lasts 54 hours (146 gr. and 129 gr. per hour reapec tively), it is obvious that if the etearine candles coat 8d. a lb., and the paraffin coat 8fd., the paraffin is really the cheaper of the two, and at 9d. would cost the same.
Some writere throw thia calculation into the form of "cost per 100 of light," where the 100, or atandard, ia taken from a atandard " hot-oil" lamp, burning every hour 815 gr. of oil, or 0.1164 lb. at Ild. a lb. Hence irt this case, the coat per 100 of light is 0.1164 x 11 = 1.2804d. Com paring this with a wax candle, buraing 125 gr. an hour, giving only the light of the lamp, and costing 2s. 6d. or 30c/. a lb., we have tie the cost per 100 of light, 125 x 11 = 1375 gr., or 0.1964 lb. x 30 = 5.892d.
Proceeding in this manner, Dr. Frankland has drawn up the followiag tablea of illuminating equivalents, or the quantities of different illuminating materials necessary to produce the saute amount of light: Young's paraffin-oil 1.00 gal. Sperm candles .. 22.90 lb.
American petroleum, No. I .. 1.26 Wax 26.40 No. 2 .. 1 . 30 Cot nposite .. 29 . 50 Paraffin candles 18 . 60 lb. Tallow 36 . 00 Taking into account the market prices of these various materiala, be concludea the comparative cost of the light, equal to that of 20 sperm candlea (each burning for 10 hours at 120 gr. an hour), to be : a. d. s. d.
Wax .. .. 7 2f American petroleum 0 6f Spermaceti .. 6 8 Young's paraffin-oil 0 5 Paraffin .. .. 3 10 Coal-gaa 0 4f Tallow .. .. 2 8 Cannel-gas 0 3 Sperm-oil 1 10 It may be remarked, in paasiog, that the difference in favour of gas, as against candlea, is very much reduced in practice, since uaers of gaa alwaya habituate themselves to a much more intense light than when they etoploy candles. Probably the same thing will obtain, mutatis mutandis, when domestic eleetric lamps are substituted for gas.
Taking into account the cost of material, its rate of consumption, ita market price per lb., and its light-power, Pcclet gives the following valuable table, upon the data that :-1 pint of oil coats 5d.; 1 lb. tallow candlea, 7d. ; 1 lb. wax candles, 2,. 2 t. ; I lb. etearine candles, ls. 4d. ; 100 ft. coal gas, 7d.; 100 ft. oil-gas, 2s. 3d. ; and that 1000 ft. coal-gas = 441 lb. sperm, or 51 lb. steaxic acid, or 6f gal. colza-oil, or 5.9 gal. sperm-oil.