The Standard Light.—The light adopted by Hurter and Driffield was the British Standard candle, burning I20 grains of spermaceti wax per hour. This candle gives satisfactory results as regards the speeds of ordinary (non-colour sensitive) plates for amateur use ; but the flame is subject to fluctuations, and obviously will give totally incorrect readings with colour-sensitive plates, on account of the spectral composition of the light, this being very yellow, or much richer in yellow, orange, and red rays than is daylight. If the candle is used for speed reading, it is always advisable to expose with the plate to be tested one of a known speed, then any variation of the speed of the latter can be allowed for in cal culating the speed of the unknown plate. It is important to see that the candle is burning regu larly at the base of the wick, with a well-formed cup of wax, free from match-heads, bits of charred wick, etc. The wick should burn over at its top to the edge of the flame, the latter being 45 mm. in height from that place where the wick begins to blacken to the tip of the flame. It is as well to make a steel metal gauge, as shown at B, for measuring this height, and it is also desirable to shield the candle flame from draughts. As it must be kept at a constant height, a retort stand with a slip arm to hold the candle, or a sliding holder, must be provided.
Another standard light, more generally used on the Continent, is the Heftier amyl-acetate lamp, a small lamp which burns pure amyl acetate that gives a flame much like the candle in spectral composition, and, therefore, open to the same objections on this score as the standard candle.
The most satisfactory standard light is acety lene, used under a pressure of about oz. The burner to be used is a Bray's " Elta," which shows no tendency to carbonise, and in which the gas escapes from two pinholes in the steatite caps. These caps contain airholes. The rod-like flames impinge against one another and produce a brilliant flat flame of about r in. diameter. Whereas in the candle the whole flame is used, in the case of acetylene it is important to screen off the margins and the tip of the flame as these flicker considerably, and it is also necessary to screen off the blue base of the flame. This can be conveniently done by a Methven screen or metal plate pierced with a square hole of 36 sq. mm. area. This screen should be as near the flame as possible (about 12, or not more than a ram.), and the bottom of the aperture should be slightly above the dark or blue area of the flame, so that it is central with the whitest and most luminous part of the flame. For exact photometric investigations, the correct position must be found by exposing a series of plates and measuring the densities, but for practical speed testing it will be sufficient if the eye, when placed at the extreme edges of the dark-slide, can see no trace of the tip, edges or base of the flame.
Renwick, of the Ilford Research Laboratory, states that there is a variation in density over the plate strips due to the variation in the intensity of the light and the small area of the flame exposed by the aperture. While this has not been confirmed by any other writer, Sheppard and Mees have confirmed the general fact that such a burner is liable to variation, and have, therefore, suggested a modification of the Pery cylindrical flame, obtained by means of a Bray burner taking •25 ft. of acetylene per hour, and constructed to give a cylindrical flame with the admixture of air. The burner is enclosed by a metal hood in which is an aperture •04 mm. and a cone reaching from this to within in. of the flame itself, the latter being 35 ram. in height.
The correct distance between the standard light and the sensitive surface should be 1 metre ; if this is not adhered to a correction must be made in estimating the intensity of the light falling upon the plate. This correction, for vary ing distances is found from Schwarzschild's formula and table, given below.
D = the distance of the light, G = the distance of any point of the plate z centimetres from the perpendicular drawn through the centre of the flame to the plate. A reference to and explanation of this table will be found under the description of the exposing instrument.
The experimenter must not overlook the danger of the intensity of the light decreasing as the gas supply and, consequently, the pressure diminish, and he should take care that the evolution of gas has ceased before exposure is made. Another point is that when the carbide receptacle is freshly charged, some air is always introduced into the gas chamber, thus leading to a decrease of luminosity of the flame. This trouble may be overcome by generating a small quantity of gas first and burning this, with the water supply cut off, and then admitting more water to the carbide till the bell is fully charged, making the exposure when the generation of the gas has ceased.
Although the acetylene light approximates more nearly to daylight than the standard candle, it is still too rich in red and orange rays, and it should be screened down with absorbent solu tions. The necessary dyes are gentian violet, acid green, mandarine orange and rose Bengal, which can be obtained in conveniently small quantities. Some pure copper acetate will also be required.
The dyes can be most conveniently made up into stock solutions as follow : A. Gentian violet . 14 grs. 2 g.
Distilled water to . 16 oz. i,000 ccs.
B. Acid green . . 7 grs. I g.
Distilled water to . 16 oz. i,000 ccs.
C. Mandarin orange . 7 grs. I g.
Distilled water to . 16 oz. i,000 ccs.
D. Rose Bengal . 28 grs. 4 g.
Distilled water to . oz. I,000 ccs.
These stock solutions must be kept in the dark. The actual filter or screen is made as follows : A solution ro rains. r cc.