Fig. 80.

336 Printing The Positive 111

Fig. 90 is a sketch of a lantern for the dark - room, a, lamp; b, ruby globe; C, white common chimney; d, tin chimney; a, plaster of Paris Joint; b, annular space for draught; e, tin collar to support globe. There seems to be a

Fig. 90.

336 Printing The Positive 112

Bromo - Gelatin Emulsion Work.

343. The double dark - slide, patented by the American Optical Company, seems veiy superior to the changing- box being lighter, surer, and enabling one to duplicate negatives quickly in case the circumstances are favorable, without having to run to the chauging - box, drop one plate and receive another, thus wafting the time when the light is in its most charming condition. The double holder is described in the Photographic Times as follows:

"As in the original slide, the plates are inserted by means of a groove - in this iaftance in the bottom. Instead, however, of both plates being placed back to back with a piece of springy paper between them, and men inserted in the wide central aperture fitted to receive them, there is a separate groove far each plate with an extra groove between them.

Fig. 91

336 Printing The Positive 113

general want of a proper lamp to develop dry plates by, and the above plan is simple, safe, and any tinsmith can make it All that is required is the ruby globe; those used by railroad companies, if deep color, are just the thing. If we could be supplied with such a lamp, or a better one, by the dealers, I think it would greatly aid dry - plate workers. - George Eastman

Table for the Simplification of Emulsion Calculations. - With a view of simplifying the calculations involved in emulsion making, Mr. William Ackland has worked out a useful table which will enable even those most ignorant of chemical philosophy to calculate with ease and rapidity the proper quantities of silver or haloid salts in any formula. Even those who are able to perform the calculations in the recognized style, will

When the end is opened ready for the reception of the plates it looks for all the world like a plate-box containing accommodation for only three plates. The two outer grooves disclosed to view are, of course, for the find their labors materially lightened by means of this table, which should be kept in a convenient place for reference in every laboratory.

Equivalents.

Weight of AgNo3 required to convert one grain of soluble haloid.

Weight of soluble haloid required to convert one grain

Weight of silver haloid produced by one grain of soluble haloid.

Weight of soluble haloid required to produce one grain of silver haloid.

Weight of silver haloid produced from one grain

AgNo3

Ammonium Bromide, . . .

98

1.734

.576

1.918

.521

1.106

Potassium " ...

119.1

1.427

.700

1.578

.633

Sodium " ...

103

1.650

.606

1.825

.548

Cadmium " . com.

344*

.988

1.012

1.093

.915

" " . anh.

272*

1.25

.800

1.382

.723

Zinc " ...

225.2*

1.509

.663

1.670

.600

Ammonium Chloride,. . .

53.5

3.177

.315

2.682

.373

.844

Sodium " ...

58.5

2.906

.344

2.453

.408

Ammonium Iodide, . . .

145

1.172

.853

1.620

.617

1.382

Potassium " ...

166.1

1.023

.977

1.415

.707

Sodium " ...

150

1.133

.882

1.566

.638

Cadmium " ...

366*

.929

1.076

1.284

.778

* These salts combine with two equivalents of silver nitrate, so that practically the real equivalent is one-half that given.

The principal bromides, chlorides, and iodides which are likely to he used in emulsions of either gelatin or collodion have been included in this table. This table presents to the reader, without any mystification which may be involved in equivalents, the actual weights of haloid or silver, as the case may be, required to convert or combine with one grain of the other. In order to test the utility of this table, let us suppose that it is desired to make, say, ten ounces of emulsion by a new formula, which, for the sake of showing the working of the table, we will write down as follows:

Bromide of Potassium,

150 grains,

Iodide of Potassium

10 "

Chloride of ammonium,

10 "

Gelatin,

200 "

Now we want to know how much silver nitrate should be employed in sensitizing this mixture. For this purpose we use the first column, in which we find against each haloid the exact quantity of silver nitrate required to fully decompose one grain. Taking, then, the figures we find in column No. 1 against the three salts in the above formula, and multiplying them by the number of grains of each used, we have the following sum:

Potassium Bromide, ....

150x1.427 = 214

Weight of silver nitrate required.

Potassium Iodide, ....

10x1.023= 10.23

Chloride of Ammonium, .

10x3.177= 31.77

or the total quantity of nitrate of silver required for full conversion, 256.00 grains.

W. P. Bolton.

reception of the plates, which are inserted face out. In the central groove is inserted a slab of blackened wood, on each side of which is a thin brass spring, which, pressing against the backs of the plates, keeps them firm against the face of their respective grooves, the wooden partition itself effectually prerenting the transmission of light from one side to the other. This partition is finished off with a thick solid piece of wood nearly the width and thickness of the whole slide, the fit-ting of it being inch as to cause it to block Dp in; perfectly light - tight manner the aperture through which the plates were admitted. In short, by the insertion of this central partition, the plates are rigidly held in their places and rendered absolutely secure from the admission of even the faintest trace of light. It contains live grooves, the two outer ones being for the sliding shutters, which may be formed of line, ebonite, or, as in the American one, a hard-pressed varnished sheet of card, or, rather, a very highly - calendered Manila hoard. These outer grooves are very thin, being only sufficient to permit the shutter to slide easily. The three others are respectively for the two plates and the central partition already described. When exposing, the shutter is pulled entirely out and is quite detached from the slide, there being a "cut-oil'" automatically thrown into action by which the aperture through which the shutter was withdrawn is instantaneously closed against the light. The above engravings will render still plainer, If possible, the verbal description just given. The grooves for both the sensitive plates and the central partition are shown at (Bcb) in the end view ). Fig. 92, of the holder. In the central groove (c) is inserted the partition (d), Fig. 93, with its side springs. That partition is so arranged that not a particle of light reaches the plate after it has been pushed into position. In Fig. 94 is shown the holder (f) with shutters (I I) partly drawn, and showing the end (G) in which the plates and divisional partition are inserted. The above engravings represent the dark slide to be much thicker than it is in reality."