INSTRUMENT FOR MEASURING THE DENSITIES OF PLATES.
The instrument devised for measuring the densities of plates is a modification of the Bunsen photometer used for measuring the illuminating power of coal gas (Fig. 562). It consists of a cubical box, A, containing the paper disc with its grease spot ; through the eyepiece, a, both sides of the disc may be viewed at the same time in two mirrors. This chamber can be drawn along a fixed scale by means of an endless cord passing over two pulleys, that on the right side being moved with a key (Fig. 563). The disc chamber is enclosed in a larger box B, which contains apertures at both sides for admitting the light from two powerful petroleum lamps. Corresponding holes are bored in the disc chamber for ad mitting the light to the Bunsen disc. The larger box is 12 in. long, 6 in. wide, and 4 in. deep ; the whole of the interior is blackened, and it is covered with a screen to exclude all extraneous light. The the two images of the disc will be alike. If a plate be now inserted, which reduces the intensity h to the intensity i, the disc chamber will have to be moved nearer to the plate in order to restore equilibrium in the two luminosities. If the distance of the disc from the centre of the instrument be now represented by y, then (1 + 1 (2) Yr = If the two equations be multiplied, a fraction is obtained which measures the opacity : I" (1 + + x kl yi opening on the left hand side is covered with a diaphragm. which reduces it to about ,1 in. diameter, and the plate to be examined is held against this opening by a couple of clips, b. On the right hand side the hole is covered with a rectangular diaphragm having an opening in. long and -1 in. wide, the length being vertical. The length of this opening can be reduced by a movable tapered diaphragm sliding in front of it, c. The apparatus is pro vided with two scales, a fixed one on the front of the instrument and a movable one on the taper diaphragm at the side. An enlarged view of this is seen in Fig. 561. The method of using the apparatus and the principle on which it is gradu ated may be briefly described as follows: In the first place it is assumed that the two lamps are equidistant from the centre of the instrument, and this distance may be represented by 1, the intensity of the light on the left is 1,, and that on the right 12 if the disc is moved to a distance x from the centre of the instrument, so that (1) _ x or Ti \ + X As the logarithm of the opacity is the density of the plate, is the density.
D = log.
b y -- log. + At the distances x y on the scale the values of log. x and log. + may , y be marked, and by reacting off these logarithms, subtracting one from the other, the density is at once obtained. For general convenience vulgar and not hyperbolic logarithms are used. By using small diaphragms any error clue to the position of the lamps is corrected, and the distance 1 can be measured be tween the centre of the instrument and the diaphragm. It is hardly necessary to
insist that great care is requisite to secure perfect uniformity of the light. With a box 12 in. long between the diaphragms, the zero point or 1 is at 6 in., and the other points in the scale are found in the following table, the graduations on both sides of zero being symmetrical, and for convenience the points thus formed are again subdivided.
The movable scale is attached to the upper edge of the taper diaphragm this is made of sheet metal, and is about 12 in. long and 2 in. wide. In the diaphragm is cut a triangular opening 101 in. long and A- in. wide at the base. The sides of this triangle must. be absolutely straight lines. This diaphragm is used for re ducing the amount of light passing through the fixed diaphragm on the right side. The zero point of the scale is marked at 10 in. distance from the apex, and the other points of the scale are marked to show directly the densities. For, at any distance x from the apex, the area of the opening and consequently the intensity of the light is reduced as 10 :: x, and the vulgar logarithm of the fraction 10 i x is the corresponding density which is marked on the scale. The following table shows the distances from the apex at which the numbers are placed: One or two examples may now be given of the method of measuring densities with this instrument. For measuring a small density the disc chamber is first moved to such a position that the images of both sides of the disc are alike ; the plate is then fixed in the opening to the left, and the sliding scale on the right is moved until the light is sufficiently reduced and the images are again equally illuminated ; the index on the diaphragm scale may then be read, and gives the density of the plate. In measuring a high density, the sliding scale is placed at zero and a piece of opal glass is fixed behind the dia phragm on the right to reduce the in tensity of the light ; the disc chamber is then moved to the right until both sides of the disc are equally illuminated. The plate is then inserted on the left, and the disc chamber moved in that direction till equality is restored; if this cannot be done with the disc chamber alone, the diaphragm on the left may also be used. Supposing the index to stand at 1'1 on the right, and afterwards at 15 to the left of the zero, then the density would be 1.1 + 1'55 = 2'65. If the index stood at on the right, and on moving the disc chamber to 1'7 equilibrium was not re stored, and the movable scale had to he placed at '75, then the density would be + 1'75 + '75 - This is a very high density, and does not occur in or dinary negatives; a density of will only allow about of the light fall ing upon it to pass. plate of density 1 allows of the light to pass, and a density of 2 allows of the light to pass, as the logarithm of 10 is 1, and that of 100 is 2.