COLOUK 115 



consequent length of the exposures necessary, 

 even at a large aperture. As to the estimation 

 of the exposure full instructions are given thereon 

 with the plates and the actinometer, but a very 

 handy method was suggested to me by Dr. F. 

 Penrose, to whom I am indebted for much valuable 

 advice on colour-work, and is as follows : When 

 using such an exposure-meter as Watkins' Standard 

 Actinometer, the duration of the exposure at /8 

 for near objects, such as sitting birds, will be the 

 same as that taken by the sensitive paper to darken 

 to the proper tint. This saves working out, and 

 tells us at once if the light is strong enough reason- 

 ably to attempt a certain piece of work. For 

 example, suppose that the sensitive paper took 

 3J seconds to darken to the proper tint, and the 

 depth of focus required necessitated stopping 

 down to /ll, then the working exposure would 

 be seven seconds a long time for most birds to 

 remain absolutely still. Often, at the opening click 

 of the shutter, the bird gives a little start, turning 

 this way and that enquiringly, trying to discover 

 the source of noise, and thus hopelessly spoiling 

 the negative. To lessen the risk of this it is well 

 to allow a bird ample time to settle down after 

 her return to the nest, and when she is quite at 

 her ease, to make a few blank exposures, opening 

 and closing the shutter to get her thoroughly 



SA 



