Photography in the Daguerreotype Process, 379 



I succeeded in constructing an instrument which I have 

 called a photographometer, the description of which appeared 

 in the Philosophical Magazine for the month of November 

 1848. 



As I have since improved it considerably, and made with it 

 a great number of experiments, I shall briefly refer to this 

 instrument, and describe the useful alterations I have made. 



In the instrument described in the Philosophical Magazine 

 for November 1848, the light struck the Daguerreotype surface 

 during the passage on an inclined plane of a metallic plate 

 having seven apertures in a horizontal line,' following the geo- 

 metrical progression 1, 2, 4, 8, 16, 32, 64; so that the Da- 

 guerreotype plate being covered with another metallic plate 

 having four series of seven holes, the effect of light through 

 every one of the seven holes was represented in proportion to 

 the opening of the moveable plate. Every one of the four 

 series of holes indicated the same number of white spots, and 

 the number of spots was the measure of the light at the mo- 

 ment. I had four series of holes in order to try several pre- 

 parations on the same plate, or to test the light on the same 

 plate at four different times. 



The improvement 1 have made consists in my being able to 

 shut every one of the holes by means of sliding blades ; so 

 that I can continue, by repeated falls, the geometrical progres- 

 sion from 1 to 512 on one plate; and when a second plate is 

 added to the double apparatus, from 1 to 8192. This enables 

 me to compare and follow the different effects of light in a 

 considerable range of intensities. This is done in the follow- 

 ing manner: — After having given one fall with all the slides 

 open, 1 shut one and give another fall, then shut the second 

 slide and give two falls, and so on, always doubling the num- 

 ber of falls for every new slide shut. 



It is by this means that I have been able to discover at 

 what degree of intensity of light the effect called solarization 

 is produced ; — on well-prepared plates of bromo-iodide it does 

 not begin under an intensity 512 times greater than that which 

 determines the first effect of mercury ; — and also at what degree 

 the decomposition producing the white precipitate without 

 mercury manifests itself, both on iodide and on bromo-iodide 

 of silver. On the first, it is 100 times quicker than on the 

 bromo-iodide; and on the last, it is produced by an intensity 

 3000 times greater than that which developes the first affinity 

 for mercury. 



The slides enable me to try the effect of different insulated 

 rays on plates affected by white light. This is done by shut- 

 ting one-half of each hole in pushing the sliding blades just 



