414 Major L. Darwin. On the Method of 



of which is one-tenth of the principal focal length of the lens, is 

 called stop No. 1; and the exposure necessary for any subject 

 with that stop is the unit of exposure for that subject. The 

 other stops are numbered in the inverse ratio of the area of their 

 effective apertures to the area of the effective aperture of stop 

 No. 1. Thus stop No. 2 gives half the intensity of illumination of 

 stop No. 1 ; and, in any case, to find the time of exposure necessary 

 to produce the same result as with the unit of exposure with stop 

 No. 1, we multiply that unit by the number of the stop in use. 

 The practical rule to find the O.I. number of a stop is to divide 

 the square of the principal focal length by 100 times the square of 

 the diameter of the effective aperture of the stop. The principal 

 focal length, which we require to know in order to calculate the 

 numbering of the stops, is found by test No. 11. 



The difficulty of introducing the O.I. numbering of stops will 

 perhaps be greater in England than on the Continent, partly because, 

 previous to the Paris Congress, the Photographic Society of Great 

 Britain had given provisional support to another system based on //4 

 as a unit. The Photographic Society has been waiting for the 

 recently published reports of the Brussels Congress to reconsider 

 this matter, and it may be hoped that they will join in the effort to 

 get the C.I. system universally adopted, notwithstanding the incon- 

 venience that must be severely felt at first by those who are there- 

 fore obliged to change their methods. 



10. Angle of Cone of Illumination with Largest Stop = , giving 



a Circular Image on the Plate of inches diameter. Angle 



of Cone outside which the Aperture begins to be Eclipsed with 



Stop C.L No. = , giving a Circle on the Plate of 



inches diameter. 



Diagonal of Plate = inches, requiring a Field of ( = 2 0) . 



Stop C.L No. is the Largest Stop the whole of the Opening 



in which can be seen from the whole of the Plate. 



If a stop, or thin metal diaphragm with a circular aperture, is 



revolved round any axis passing through its plane, and if it is 



regarded from a little distance, the whole aperture, foreshortened of 



course, can be seen except in one position in each half revolution ; if 



m a similar way a piece of tubing is revolved about an axis at 



ght angles to its own axis, there is only one position in which 



J whole of the aperture can be seen, and any movement from 



one position will cause the opening to begin to be eclipsed, 



giving it a lozenge-shaped appearance; as the movement goes 



on, this opening will get smaller and smaller till it is quite obli- 



ed. In looking through a lens as it is revolved about an 







