THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 299 



a short distance from the k^ns, the object of which is to cut off all 

 those rays of liijht which do not come directly from the object to 

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

 that which is to the right in nature being to the left in the photo- 

 graph. This can be remedied by using a 

 mirror outside, as K J, in figure 9. This 

 arrangement, however, reduces the quan- 

 tity of light and increases the time of the 

 operation one-third. It will, of course, 

 be adopted 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- 

 s' 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 no impression is visible upon the tablet when it is withdrawn from 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 8 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, jiccording to the season. Objects in shadow, 

 even during the brightest weather, will require twenty minutes to 

 be correctly delineated. From what has been stated it will l)e evident 

 that it is impossible to fix with any precision the exact length of 

 time necessary to ol)tain 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 sunn}' climes of Italy and southern France these designs nuiy 

 be obtained much more [)rom})t]y than in the uncertain climate of 

 Great Britain. It is very iin})ortant that the time necessary is not 

 exceeded — prolonged solarization has the effect of blackening the 



