Page:EB1911 - Volume 21.djvu/552

Rh at a given angle with the normal must be less than that sent off in the direction of the normal in the inverse ratio of the areas of the corresponding normal and oblique elements, that is, as the cosine of the given angle to unity. For most practical purposes, and so long as the obliquity is not great, Lambert’s law may be assumed to hold.

In almost all accurate methods of photometry the aim is to bring the illuminating powers of the two sources to equality. This may be effected by altering the distance of either light from the illuminated surface. Or we may use polarized light and diminish the intensity of the stronger beam by suitable rotation of a Nicol prism, a method particularly useful in spectrophotometer’s. The same result may also be effected by interposing absorbent disks, the precise absorbing powers of which must, however, be known with great accuracy. Another useful method is that first described by H. Fox Talbot in 1834, and used with effect by Professor William Swan (1849), and more recently by Sir W. de W. Abney. Talbot’s law is thus enunciated by H. von Helmholtz: “When any part of the retina is excited by regularly periodic intermittent light, and when the period is sufficiently short, the resulting impression will be continuous, and will be the same as that which would be produced if the whole light were distributed uniformly throughout the whole period.” Talbot deduced the principle from the well-known experiment in which a continuous luminous line is produced by rapid rotation of a luminous point. If the principle be granted, it is obvious that any mechanism by which a ray of light is obstructed in a regularly rhythmic manner during definite intervals t&#8202;′, separated by intervals t, during which the light is allowed to pass, will have the effect of reducing the apparent brightness of the ray in the ratio t/(t + t&#8202;′) This is frequently accomplished by placing in the ray a rotating disk perforated by radial sectors, the so-called Talbot disk.

If photometric results are to be of general value it is essential to have a unit in which to express all other intensities. For example, electric lights are classified according to their “candle-power.” The candle, in terms of whose brightness the brightness of other sources of light is to be expressed, must, of course, fulfil the conditions demanded of all standards. It must give under definite and easily realizable conditions a definite and constant luminous effect, and it must be easily reproducible. The earlier attempts to get a candle of constant brightness were not very satisfactory. The British standard is a sperm candle which weighs ℔, and loses in burning 120 grains per hour. It is found that these conditions are not sufficient to determine the luminous power of the candle, since the length and shape of the wick, the height of the flame, and the composition, temperature and humidity of the atmosphere all have an effect upon its brightness. The same is true of other similar sources of light—for example, the German standard candle, which is made of paraffin, has a diameter of 2 cm, and has its wick cut until the flame is 5 cm. high, but which with all precautions suffers continual alterations in brightness. For ordinary practical purposes, however, these candles are steady enough. Other kinds of flame have also been used as a standard source of light. The oldest of these is the French Carcel lamp, which is provided with a cylindrical Argand burner, and gives the standard brightness when 42 grammes of colza oil are consumed per hour. The supply and draught are regulated by clockwork. A. G. Vernon-Harcourt’s pentane standard, in which a mixture of gaseous pentane and air is burnt so as to maintain a flame 2.5 in. high at ordinary barometric pressure, gives good results, and is readily adjustable to suit varied conditions. Several forms of this standard have been constructed, one of the most important being the 10 candle-power pentane lamp, in which air saturated with pentane vapour is burnt in a specially-designed burner resembling an Argand burner. For photometric purposes a definite length of the lower part of the flame is used, the upper part being hidden within an opaque tube. The amyl-acetate lamp designed by H. von Hefner-Alteneck has been elaborately studied by the German authorities, and at present is probably more used than any other flame for photometry. It is of simple construction, and gives the standard

brightness when it burns with a flame 4 cms. in height in still air of humidity 0.88% and free of carbon dioxide. The presence of carbon dioxide and increase in the humidity have a marked effect in diminishing the brilliancy of the flame. If the vapour pressure is e and the barometric pressure p, the strength of the flame, when all other conditions are fulfilled, is given by the formula

1.049 − 5.5e/(p−e)

One disadvantage for photometric purposes is the reddish colour of the flame as compared with the whiter artificial lights in general use.

For an interesting account of the various experimental investigations into the properties of the Hefner flame see E. L. Nichols, “Standards of Light,” Transactions of the International Electrical Congress, vol. ii. (St Louis, 1904). Ångstrom’s determination of the radiation of the flame in absolute energy units is also of special interest.

Attempts have been made, but hitherto with limited success, to construct a convenient standard with acetylene flame. Could a satisfactory burner be devised, so that a steady brilliancy could be easily maintained, acetylene would, because of its intense white light, soon displace all other flames as standards.

J. Violle has proposed to use as standard the light emitted by a square centimetre of surface of platinum at its melting-point, but there are obvious practical difficulties in the way of realizing this suggested standard. J. E. Petavel, who carefully examined the necessary conditions for producing it (Proc. Roy. Soc. 1899), finds that the platinum must be chemically pure, that the crucible must be made of pure lime, that the fusion must be by means of the oxy-hydrogen blow-pipe, that the gases must be thoroughly mixed in the proportion of 4 volumes of hydrogen to 3 of oxygen, and that the hydrogen must contain no hydro-carbons. Under these conditions the variation in the light emitted by the molten platinum would probably not exceed 1%. O. Lummer and F. Kurlbaum have proposed as a standard a strip of platinum foil 25 mm. wide and .015 mm. thick brought to incandescence by an electric current of about 80 amperes. The temperature is gradually increased until th of the total radiation is transmitted through a water trough 2 cm. in width. This ratio is determined by means of a bolometer, and so long as it is adjusted to th the light is practically constant.

For comparative photometric work the incandescent electric light is very convenient, having the one great advantage over candles and flames that it is not affected by atmospheric changes. But it does not satisfy the requirements of a primary standard. It ages with use, and when run at constant voltage gradually loses in brilliancy, partly because of changes in the filament itself, partly because of the deposit of carbon on the interior of the bulb. Professor J. A. Fleming has shown that very good results can be obtained if carbon filaments carefully selected

and run in ordinary bulbs for a definite time at a little above their normal voltage are remounted in large clear glass bulbs 6 or 8 in. in diameter. If used sparingly, and never above their marked voltage, these large incandescent bulbs have been found to remain constant for years, and therefore to be eminently suitable as secondary standards. In his Handbook for the Electrical Laboratory and Testing Room (vol. ii) Fleming concludes that the best primary standards are the Violle incandescent platinum and the Vernon-Harcourt pentane one-candle flame; and that the most convenient practical standards are the Hefner lamp, the ten-candle pentane lamp, and the Fleming large bulb incandescent electric lamp. Comparisons of the intensities of these various standards do not give quite concordant results. Thus three different authorities have estimated the 10-candle pentane lamp as being equal to 10.75, 11.0, 11.4 Hefner lamps.

A specially constructed instrument or piece of apparatus for comparing light intensities or illuminations is called a