FLIGHT. 107 



is a very different matter, and there are many other 

 factors all tending to accentuate the difficulty. 



Much of this work is done in the neighbourhood 

 of sea-cliffs, as the habits of the birds frequenting 

 them are especially favourable, and the light is there 

 very strong : in fact, there is at times a surplus of 

 light over and above what is absolutely necessary, 

 even after allowing for the great speed at which 

 the shutter has to work. That is to say, if our 

 judgment decides that l/250th part of a second 

 is a legitimate speed at which to attempt a certain 

 piece of work so far as movement is concerned, the 

 actinic condition of the light may be such that the 

 ultra-rapid plate in use would be by this fully or 

 even over exposed. When this occurs there are 

 two ways of utilizing this very desirable surplus 

 of light. On the one hand the lens may be stopped - 

 down, thereby increasing the depth of focus a 

 thing of much value in this work or on the other, 

 the speed of the shutter may be increased to try 

 to avoid movement in the image. For several 

 reasons the latter is to be preferred, for by increas- 

 ing the shutter's speed and leaving the lens at or 

 near full aperture, more light is thrown up on to 

 the focussing screen : and this aids us much in 

 instantly determining whether or not the image is 

 sharp and the grasping of this as the bird sweeps 

 by is one of the chief difficulties in the work. On 



