114 PHOTOGRAPHY FOR BIRD-LOVERS. 



with transparencies, but these, if the exposure 

 has been correct, are of such astonishing beauty as 

 to be doubly worth the trouble expended on them. 



So far as concerns the field-manipulations, all 

 that has been said before applies now with equal 

 or even greater force. A hiding-tent is indis- 

 pensable, and it will have to be rather larger than 

 that generally in use, in order to allow of some little 

 necessary movement without the tent being shaken. 

 There should also be a little window or aperture 

 at the back where, out of sight of the bird, we may 

 from time to time test the light. The entrance slit 

 may serve this purpose if it is not too low, but a 

 small hole towards the top is to be preferred. It 

 is necessary to keep testing the light as its actinic 

 value often changes considerably from hour to 

 hour, and even more rapidly on windy days with 

 much moving cloud overhead ; and a correct expos- 

 ure is most essential. To help us to gauge the value 

 of the light, an actinometer is indispensable. Those 

 meters which employ sensitive paper are the most 

 satisfactory, but the ribbons or discs in them must 

 not be allowed to get stale, and it is better to 

 change them about every two months in summer, 

 whether they are exhausted or not, than to run 

 any risk of working with stale paper. 



The special difficulty in taking bird-autochromes 

 lies in the extreme slowness of the plates and the 



