5*

is found to be firmly cemented to its support, place it in the dish, and pour in water until it reaches a quarter of an inch up the side of the mirror (Fig. 25). Measure this quantity, as it will show the amount of silvering solution required to be prepared. Now proceed—

To Clean the Mirror.

To do this fill the end of the test tube with cotton wool, leaving plenty outside the tube (Fig. 24), and having poured a small quantity of strong nitric acid on the front of the mirror, rub the acid well all over the front and sides with the cotton wool brush. Now pour on the surface a little water to dilute the acid, and again well rub the front and sides. Place the speculum under a water-tap for a few minutes, until the acid is washed away, and finally well rinse with distilled water; then place it in the silvering vessel (previously thoroughly cleansed) and pour in distilled water until it touches the entire face of the speculum.

The water will appear to cleave to the entire surface when it is perfectly clean. If the operator fails to completely clean the mirror with the acid, he should, after the acid is entirely washed away, dry the surface with a perfectly clean linen cloth, and then apply a mixture of equal parts of a solution of potash (1 oz. in 10 oz. distilled water) and alcohol, well distributed with a clean cotton wool pad; this mixture is then diluted by adding a little water, and again rubbed; finally the surface is plentifully rinsed with distilled water, and the mirror is then supported face downwards in a vessel of distilled water, as before described. In very cold weather it may be found advantageous to have the mirror slightly warmer than the bath, and both these warmer than the silvering vessel, which should be kept cool. The mirror may be warmed by standing it in distilled water at a temperature of about 90°, any dish except the silvering vessel being employed to hold it.

To Immerse tlie Mirror.

Remove the mirror from the distilled water, and wipe the back and edge with some clean cotton wool if there is any water thereon, taking care not to touch its surface. If this precaution be not taken the water is liable to drain down the sides of the mirror whilst silvering, and cause streaks at the edge of the film. Having poured

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La langue de reconnaissance de l'OCR est l'Anglais.

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