The glasses for photography are sold in certain fixed sizes. When new, the sharp edges must be smoothed over with a corundum file; then carefully wash, rub with a soft rag, finish with chamois leather. When the glasses have been used, they are more difficult to clean. If they have been varnished, they must be soaked in a solution of common soda, or carbonate of potash till the varnish peels off. If the carbonate of potash or common soda ,does not bring off the varnish quickly enough, use a solution of an ounce of nitric acid to every half-pint of water. Apply this to the glasses with a piece of cotton wool, fixed on a handle, so as to avoid contact with the nitric acid, which stains the hands. When the plates have been well covered with any of these solutions, let them stand to drain in a rack, then rub and wash well with a sponge and water. Dry. The side intended for the collodion must next be polished with Tripoli powder and a few drops of spirit of wine, rubbed over with cotton wool, wipe off the excess of Tripoli, and polish with a dry chamois leather. Place the cleaned plates into a properly grooved box, with all the faces prepared for the collodion turned one way.

It is advisable to wash all glasses as soon as possible after use, as by not doing so the varnishes dry on very firmly, and are difficult to remove. Waste collodion may be utilised for cleaning glasses; it removes all grease. When glasses have once been cleaned, avoid touching them with the naked hand, as it is sure to leave stains. There are various holders in use; the rubber pneumatic is one of the best. Before using a plate, dust it carefully with soft silk or a piece of clean old rag. Perfect cleanliness is imperative. Sizes of Photographic Glasses. - 2 1/2 in. x 2 in., ninth plate; 3 1/4 x 2 3/4, sixth; 4 1/4 x 3 1/4, quarter, carte de visite; 5 x 4, third; 6 1/2 x 4 3/4, half; 8 1/2 x 6 1/2, whole. All plates above whole size are denoted by dimensions only, 6§ in. x 3 1/4 in. stereoscopic plate. The following are the diameters and focal lengths of lenses suitable for portraits of the usual sizes: -

Diam. of Lens.

Focal length.

Size of picture.

in.

in.

in.

1 1/4

3 1/4

2 1/2

x

2

2 3/8

5

4 1/4

x

3 1/4

3 1/4

7

5

x

4

2 3/8

7

6 1/2

x

4 3/4

3 1/4

9 1/2

8 1/2

x

6 1/2

For groups: -

3 1/4

11

9

x

7

4 1/4

13

10

x

8

4 3/4

15

12

x

10

5 1/4

19

15

x

12

For views: -

1 3/4

8

6

x

5

2

10

7

x

6

2 3/8

14

9

x

7

3

16

12

x

10

4

24

16

x

12

The dimensions of pictures given are maximum sizes, and to ensure a thoroughly good picture, it is best to use a lens of a larger size than is absolutely necessary. Patent plate glasses are the best for negatives, although flatted crown and sheet glass may be used.

Positives are sometimes taken on deep red or purple coloured glass. Whatever kind of glass is chosen, it should be as flat as possible, otherwise it will be difficult to place in the dark slide.