Photometric Observations of the Sun and Sky. 2 5- 5 



which it commenced, i.e., perpendicular to the axis ; but when the 

 steel is soft, the plane of the tear gradually tilts over and coincides 

 with the surface of least resistance to shearing, i.e., becomes inclined 

 at 45 to the axis. 



Now, at rupture, an originally soft bar is harder in the centre of 

 the narrowed section than at the circumference, where the drawing 

 out has been less ; hence, fracture commences at the centre perpen- 

 dicular to the axis, and tears outwards until it reaches the softer 

 material, when it will continue along a surface of least resistance to 

 shearing, i.e., along a surface formed by the intersection of two cones. 

 Hence, we find the fracture of a soft steel bar consisting of a crater 

 with a more or less extended base ; see figs. 5 and 6, Plate 2, and 6 

 and 7, Plate 3. 



The harder the steel, at the outset, the broader will be the base of 

 the crater, until, in very hard steels, there is only a rim or crown left 

 round the edge ; and in the hardest steels all trace of the surface of 

 least resistance to shearing disappears. 



[Note. I have employed the term " hard " in the sense usually 

 understood, i.e., where the " hardness " is measured by the value of 

 the limit of elastic resistance.] 



"Photometric Observations of the Sun and Sky." By 

 WILLIAM BRENNAND. Communicated by C. B. CLARKE, 

 F.R.S. Received October 30, Read December 11, 1890. 



1. In the publications of the Society from 1859 to 1870, many com- 

 munications by Sir Henry Eoscoe on this subject will be found. Of 

 these, the most important bearing directly on my observations 

 are 



a. Bunsen and Roscoe, " On the Direct Measurement of the 

 Chemical Action of Sunlight," in ' Phil. Trans.,' 1863, pp. 139-160. 



It is proved, inter alia, that equal shades are produced in photo- 

 graphically sensitised paper by equal products of intensity of light 

 X time of insolation. The preparation of a photographic paper which 

 shall always possess the same degree of sensitiveness is carefully 

 described. 



b. Roscoe, " On a Method of Meteorological Registration of the 

 Chemical Action of Total Daylight," in Phil. Trans.,' 1865, pp. 605- 

 681 [Bakerian Lecture]. 



The law is stated, inter alia, that light of intensity 50 acting for 

 L second has the same effect as light of intensity 1 acting for 

 50 seconds. 



The mechanical arrangement for exposing the paper horizontal, 

 or by the aid of a vertical drum, is explained. 



