AUGUST 1949 DARK -ROOM 59

towards the shorter waves, owing to the visualin one plane perpendicular to the axis; see fig. 5. task of the being transferred from the conesThus the scanty light is emitted mainly in the axial to the rods in the (Purkinje effect). Fordirection of the lamp, that is to say downwards the brightnesses that one could have in the darkwhen the lamp is mounted in the normal position. room we therefore have to reckon with a reducedIn order to get some light at the side - the lamp sensitivity of the eye for the red 6). has to serve as "beacon" for orientation of the not It appears that for general lighting when dealingyet adapted eye - the bulb is frosted on the inside. with panchromatic emulsions it is best to choose a Notwithstanding the very small flux, this light spectral area near the maximum of the shiftedhas to be used with care, as will be understood relative luminosity curve, i.e. near 5050 A. Thatfrom the foregoing. The panchromatic film must be is why for this purpose a dark -room safe -exposed to it as little as possible, certainly less light has been made, the maximum radiation lyingthan 20 seconds at such a distance from the lamp around 5600 A. The maximum cannot be broughtthat the illumination is 0.0005 lux (about 2 metres much closer to the said optimum wavelength be- when the flux is 0.01 lumen). By way of comparison cause the spectrum must inevitably extend towardsit is to be noted that the illumination of the earth still shorter waves and there the sensitivity of theon a clear starlit night with no moon averages emulsion (in comparison to that of the eye) begins about 0.0003 lux 7). to increase considerably. The flux of this lamp is much smaller than 0.1 lumen. The developing of positive paper Formerly this lamp was made in the form of a As explained in the introduction, dark -room light- 5 W incandescent lamp with a bulb of coloureding is fundamentally difficult for emulsions which glass ("natural glass"). Now, like the dark red and while having high general sensitivity tend to approx- other lamps stillto be mentioned, the bulb isimate the relative lunimosity curve of the human covered with a coating of coloured lacquer. Thiseye. In the case of positive papers neither one nor simplifies manufacture and closer tolerances canthe other is essential. In principle the general be prescribed for the transparency. It proved to besensitivity need not be high, since during the expo- difficult, however, to get the desired small fluxsure there is no movement and the paper can there- for the green lamp with a filament in a lacqueredfore be exposed as long as desired. Further, the col- bulb, since the layer of lacquer has to be so thickour sensitivity can quite well be limited to a small that it cannot safely dissipate the power it has topart of the , say violet and , absorb from the filament; in course of time itsince the negative is in any case colourless. The would peel off. For this reason instead of the in-general sensitivity should, it is true, not be too candescent lamp a glow lamp filled with argon is now used. The argon spectrum contains lines hav- ing wavelengths favourable for the purpose; the undesired spectral lines are intercepted by the green lacquer. This can be seen in fig. 4f. The power of this lamp is about only 1 W, so that the difficulty described above does not occur. The electrodes are made in the form of two semicircular discs lying

6)It may be surprising that in the experiments regarding the perception of contrasts in the negative a sensitivity curve was found corresponding approximately tothe relative luminosity curve of the eye at high levels of brightness (fig. 1), although the level of illumination in those experi- ments was already in the Purkinje range. A possible explanation for this is that when one is looking sharply at an object it is particularly the central part of the retina (the fovea), where there are almost exclusively cones, that is brought into action. The sensitivity of the eye does not then depend upon the level of brightness. H. Arens and J. Eggert (Z. wins. Phot. 24, 229-248, 1926) have Fig. 5. Construction of the green dark -room safe -light for made a study of the consequences of the Purkinj e effect general lighting when developing panchromatic emulsions. for dark -room illumination, but without differentiating It is a green -lacquered argon glow lamp with disc -shaped between the criteria for the general lighting and thoseelectrodes perpendicular to the axis of the lamp; the lacquer for the examination of the negative. A separation of the has been partly removed to show the electrodes. The bulb is lighting for these two objects, making allowance for the normally frosted on the inside. Purkinje effect, has been proposed specially for the developing of X-ray films by F. Luft and M. Biltz, Trans. Ill. Eng. Soc. Japan 25, 101-112, 1941. 7)P. J. Bouma, Philips Techn. Rev. 5, 297, 1940.