Prof. H. W. Dove on a New Photometer. 25 



dence I could only extend to a distance of 160 feet the experi- 

 ments to ascertain if, by parallelism of the rays, the action of 

 distance were eliminated or not ; but the great intensity of light 

 convinced me that the test was applicable to very considerable 

 distances. Experiments at a constant distance of a stationary 

 light might give information as to the limits of the influence of 

 incipient obscurations in the atmosphere. Here the instrument 

 acts as a diaphanometer. I also think that in solar eclipses it 

 furnishes a much more accurate mode of determining the illu- 

 mination than previous methods. 



The intensity of the light of different parts of the spectrum 

 may be obtained in two ways, — by allowing the individual rays 

 to fall directly on the microscope arranged horizontally, or by 

 directing the microscope upon a white wall on which the spec- 

 trum is received. A comparison of both shows the influences of 

 diffusion caused by rough surfaces, and dependent on the length 

 of wave. In this case I think it convenient to use, for illumina- 

 ting the front side of the photograph by reflexion, the solar light 

 itself, incident through another aperture in order to eliminate 

 thereby the varying intensity at different times according to the 

 transparency of the air. 



I have investigated polarized colours as follows : — Between 

 two Nicols, selenite plates of J to ^ thickness were interposed, 

 and the photographic picture was laid on the aperture of the upper 

 one. In the polarizing apparatus which I have described, this 

 takes place simply, inasmuch as, after introducing the selenite 

 plates, it is examined like a telescope. The bull's-eye of the ap- 

 paratus concentrates a luminous flame on the aperture of the po- 

 larizing Nicol. 



Determination of the Light in a given space. 



If the horizontal microscope be placed in any part of the room, 

 the quantity of light which falls from above upon the object 

 decreases with increasing distance from the window, while that 

 sent upwards from the illuminating mirror directed towards the 

 sky remains the same. It is hence clear that the equilibrium 

 established in one place between the upper and lower illumina- 

 tion ceases in another, and in this way the equally bright places 

 in the room are obtained. 



For photographic and, as I have heard, for ophthalmological 

 purposes, it is often desirable to have a definite illumination in 

 a room. I will assume that on a day of given brightness a pho- 

 tographic view gives the desired result in a given time. The 

 microscope is placed vertically near the apparatus, and a flame 

 removed so far from the illuminating mirror that compensation 

 is produced. To obtain the same light on repeating the process. 



