64 NOTES AND MEMORANDA. 



by means of dry plates. If a considerable number of small cassettes 

 are prepared, you can provide them with the dry plates in the evening, 

 and the next day, as wanted, take inicro-photographs. These, for 

 which according to the intensity of the light one second up to several 

 minutes are required, may be let stand till the evening if you are not 

 in a position to darken your room, in order that you may at one and 

 the same time develop and fix the pictures. The time required 

 dejiends of course on the source of light. If direct sunlight is 

 used with a low magnifying power, a perfect photograjjh may 

 be produced in a fraction even of a second. With a magnifying 

 power of 200 to 500 diameters, the time of exj^osure required with sun- 

 light is from twenty-five seconds to half a minute, under some circum- 

 stances a whole minute. Bright daylight or sunlight reflected from a 

 bright cloud requires even with low powers fi'om a half to two and 

 even three minutes. High powers cannot be used in diffused daylight. 

 Magnesium light offers a good substitute, and by employing it with 

 Wilde's dry plates superior " photograms " are obtained even with very 

 high amj)lification. The time of exposure is in proportion to the 

 intensity of the light. Magnesium light is about forty times weaker 

 than sunlight, consequently the time of exposure required for a 

 photograph with magnesium light is about forty times as long as 

 with direct sunlight : for low powers therefore a period of about 

 three-quarters of a minute, for high jjowers a 2)eriod of from several 

 minutes to a quarter of an hour. The latter period is requisite with 

 the highest i:)owers which as yet it has been possible to employ for 

 photograj)hic purposes. 



The further development and fixing of the image is effected 

 according to Wilde's directions in the following manner : — 



First, over the plate which has received the impression there is 

 poured, to develop it, a solution of 



20 cubic centimetres alcohol, 

 5 „ „ distilled water, 



10 droijs of the solution B (below) of bromide of potash, 



and it is left to the action of this mixture one to two minutes; then it 

 is carefully rinsed with water till all greasy streaks have disappeared, 

 and the water flows quite evenly over the plate. 



In the development the following solutions are wanted : — 



A. 5 grammes pyrogallic acid, 25 cubic centimetres alcohol, 25 cubic centi- 

 metres distilled water. 



B. 5 grammes bromide of potash, 75 cub. ceutim. distilled water. 



C. 3 grammes gelatine, 20 cub. ceutim. of acetic acid, 400 cub. centim. dis- 

 tilled water. 



D. 25 grammes of carbonate of ammonia, 150 cub. centim. of distilled water. 



Shortly before it is wanted for use a mixture is made of 



40 drops of A. 



20 „ B. 



10-15 „ C. 



15 cub. cm. D. 



The latent image is washed over with this mixture and kciA moist 

 for several minutes by repeated washings. 



