8 REPORT— 1839. 



the degree of obscuration of daylight, produced by a high wooden wall 

 or hoard, recently erected in a garden behind two valuable houses in 

 George Street, Hanover Square, London. In the latter case he had 

 recourse to a photometric plan, which appears to be free from all am- 

 biguity or source of error. The chloride and several other salts of 

 silver are so very sensitive to light, as to take a dark grey tint from 

 exposure to diffuse daylight in a very short time. When the aspect of 

 the sky continues uniform for two or three hours, paper imbued with 

 the nitrate, carbonate, chloride, or phosphate of silver, will assume a 

 depth of grey or brown tint proportional to the time of its free exposure 

 to the day. Availing himself of this principle. Dr. Ure simultaneously 

 placed pieces of paper so prepared, in the apartments subject to the 

 darkening influence of the wall, and in apartments of the adjoining 

 house, not under that influence. The papers which enjoyed the free 

 aspect of the sky, having assumed a decided depth of hue in half an 

 hour, he folded them up from the light, and proceeded to watch the 

 papers placed opposite to the windows less or more obscured, till he 

 observed them to take the same depth of tint. The relative degrees 

 of diurnal illumination in different apartments of any house — in diffe- 

 rent countries — or on different days in the same place or country, may 

 thus be accurately measured and permanently registered by a series of 

 photogenic impressions of any form, which will exhibit the progressive 

 depths of tint, after an exposure during a certain number of minutes to 

 diffuse daylight ; care being always had to prevent the direct or reflected 

 impulsion of the sunbeams. The tints thereby produced being fixed 

 by water of ammonia, hyposulphite of soda, or any of the well-known 

 photogenic expedients, will serve as standards of comparison to enable 

 us to estimate the vivacity of daylight in any region of the globe, from 

 the time in which paper similarly prepared with a standard salt of silver, 

 acquires from exposure to daylight the same hue. And since the com- 

 parison of tints may be made with considerable precision by an expe- 

 rienced eye, this photometric method may prove a valuable addition to 

 the scientific resources of the meteorologist. 



On the use of the Oxy-Hydrogen Microscope in exhibiting the 

 phcBnomena of Polarization. By J. F. Goddard. 



In the description of Mr. Goddard'spolariscope, published in the Trans- 

 actions of the Society of Arts, the polarizing mirror will be found to be 

 placed after the third condensing lens, which is only used with the high 

 magnifying powers ; this arrangement he was compelled to adopt, as the 

 microscope which had been previously made would not admit of any 

 other. But he has since had one constructed, in which he could intro- 

 duce every improvement that his experience suggested ; and one of 

 the most important was to place the polarizing mirror much nearer the 

 light, so that he can now use, with the polariscope, the lowest magnify- 

 ing powers, and consequently exhibit much larger illustrations and de- 

 signs in selenite ; also the different forms of unannealed glass ; and not 



