526 Easy Method of measuring Intensity of Total Daylight. [June 18. 



as shown in the figure. On A B is marked the scale, beginning with zero 

 at A. The tube is pushed over the rod till the extremity C coincides with 

 the zero line at A. 



The method of using this instrument is as follows :— .Draw the tube 

 gently up tlie rod, at the same time looking steadily at the black spot 

 before mentioned. It will be found, as the tube ascends, that the black 

 spot will gradually disappear, and ultimately vanish in the gloom ; it will 

 also be found that on dif event days, and different hours of the same day, 

 the point at which the black spot vanishes will vary with the intensity of 

 the light. This point is read off on the graduated scale, and thus we are 

 enabled to measure the intensity of the light at any required time. In 

 taking an observation, it would be well to state whether that portion of sky 

 round the zenith from which the cone of rays proceeds be clear or cloudy. 



It will be seen that the result obtained by this method is not scientifically 

 correct, as it will be affected by the eyesight of the person who makes 

 the observation, but only in a slight degree. The method of measuring 

 light, as just described, has been known to me for upwards of three years. 

 The hope that I should some day be enabled to make the instrument 

 scientifically correct has hitherto prevented me from making it public. 

 As I understand that it is highly desirable to have some means of esti- 

 mating the changes in the light which will occur during the total eclipse 

 of the sun in August next, I no longer feel justified in keeping in the back- 

 ground an instrument which maj possibly be of some slight assistance. 



