THE SCREEN AS AN AID TO BRILLIANCY.
Thus the screen may be used to obtain brilliancy quite apart from a considera tion of colour values, as, for example, when photographing mountain scenery, distant views, or in a veiled atmosphere. The shadow light under such ciremn stances consists largely of blue rays, which the screen is able to reduce, and thus enable the yellower high lights to impress themselves on the plate. Prob ably nothing will make this so clear as a practical experiment. Let two plates, which should preferably be orthochro matic, be exposed upon a distant view, or one slightly obscured by mist. The first should be exposed in the ordinary way without a screen, and for a correct length of time, and the other simul taneously under precisely the same condi tions, except that it should be exposed through a fairly deep yellow screen, and for a proportionately increased time. Of course, it is understood that the sub ject should be the same in each case. The greater brilliancy of the latter negative will, if the experhnent is properly carried out, be very apparent. It must, however, be understood that this procedure is not always advisable. It is, indeed, a power which is very liable to be abused. From
an artistic point of view a slight indis tinctness of distant objects is essential, in order to secure that atmospheric per spective which is, or should be, one of the chief charms of photographic views. By over-correcting with the screen, dis tant mountains may be rendered with a harsh distinctness which is exceedingly unsatisfactory, and dwarfs them into mole hills. A fact which should not be lost sight of is that, when photographing in a yellow light, a screen is usually un necessary, and may even be harmful. It will be seen that there are certain aesthetic considerations to be taken into account besides those of scientific interest. So that, in order to come to a definite con clusion as to the choice of plates and screens, it will be necessary to consider them in connection with the special work for which they are to be employed.
immediately before it. In small size cameras the filter may be slipped into the dark slide and focussing done through it on a piece of ground glass placed in the slide. This answers well enough for occa sional exposures, but for general use the method L,hown in Fig. 598 is far preferable.