SUNSHINE RECORDER. 359 



the length of the part of the paper actually burnt as compared 

 with the rest of the track that the focus would have followed and 

 burnt had the sun shone all the time. 



Somewhat analogous appliances are also used where the record- 

 ing action is the production of chemical changes in photographic 

 paper, &c., by the sunlight falling thereon. When the sunlight 

 falls fully on properly prepared silvered photographic paper (Expt. 

 392) a large amount of darkening and blackening is produced ; but 

 when the sun is more or less obscured, the amount of darkening 

 is proportionately lessened. In sunshine recorders of this kind 

 it is usually not necessary to condense the light by means of a 

 lens ; a slit is made in a plate of metal, &c., and underneath this 

 a strip of prepared paper is made to pass regularly by clockwork, 

 being unwound from one roller and wound up on another, some- 

 what as the painting of a panorama exhibited to an audience. 

 Thus suppose 12 inches of paper pass under the slit during 12 

 hours, and the sun shine at intervals during the time, the particu- 

 lar hours of the day when the sun was obscured will become 

 visible by noticing whereabouts in the course of the 12 inches the 

 light parts lie ; whilst the relative intensity of the light is indicated 

 by the greater or less amount of blackening produced at the 

 darkened parts of the paper. 



Automatic photographic registering appliances of this descrip- 

 tion are now largely used in meteorological and other observatories 

 for the purpose of recording the height of the barometer, the 

 fluctuations of the thermometer, the variations of the compass- 

 needle, &c., &c. 



CHAPTER XXV. 



RADIANT CHEMICAL ACTION OR ACTINISM : PHOTOGRAPHY. 



It has already been stated that whilst the most highly refrangible 

 rays have the power of bringing about chemical changes to the 

 greatest extent (Pa. 351), still the rays of lower refrangibility are not 

 by any means destitute of this property. The amount of chemical 

 change brought about by one kind of ray as compared with 

 another is by no means the same for all kinds of substances. Just 

 as a given coloured transparent medium absorbs visible radiations 

 of certain kinds more freely than others, so that these are more 



