THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY, 299 



a short (lisliincc froin the lens. {\w object of which is to cut otf all 

 those raj's of li<;ht which do not come directly from the object to 

 which the camera is directed. This instrument reverses the objects, 

 that Avhich is to the rit,dit in nature being to the left in the photo- 



grai)h. This can be remedied by using a 

 mirror outside, as K J, in figure 1). This 

 arrangement, however, reduces the quan- 

 tity of light and increases the time of the 

 operation one-third. It will, of course, 

 I)e ad()[)ted only when there is time to 

 spare. 



After having placed the camera in 

 front of the landscape or any object of 

 which we desire the representation, our 

 first attention must be to adjust the 

 plate at such a distance from the lens 

 that a neat and sharply defined picture 

 is produced. This is, of course, done by 

 the obscured glass. The adjustment being 

 satisfactorily made the glass is removed 

 and its place supplied by the frame con- 

 »• taining the prepared plate, and the 



whole secured by screws. The doors are 

 now opened by means of the half circles and the plate exposed to 

 receive the picture. The length of time necessary for the production 

 of the best effect, varying with the quantity of light, is a matter 

 which requires the exercise of considerable judgment, particularly as 

 no impression is visible upon the tablet when it is withdrawn from 

 the camera. At Paris this varies from three to thirty minutes. The 

 most favorable time is from 7 to ;^ o'clock. A drawing which in the 

 months of June and July may be taken in three or four minutes will 

 i-equire five or six in May or August, seven or eight in April or 

 September, and so on, according to the season. Objects in shadow, 

 even during the brightest weather, will recjuire twenty minutes to 

 be correctly delineated. From \n hat has been stated it will be evident 

 that it is impossi]>le to fix with any precision the exact length of 

 time necessary' to obtain photographic designs, but by practice we 

 soon learn to calculate the required time with considerable correct- 

 ness. The latitude is, of course, a fixed element in this calculation. 

 In the sunny climes of Italy and southern Fi-ance these designs may 

 be obtained much more pronii)tly than in the uncertain climate of 

 Great Britain. It is very important that the time necessary is not 

 exceeded — prolonged solarization has the effect of blackening the 



