52 An Apparatus to illustrate the Formation of Volcanic Cones. 



cient of absorption is so large. In the case of the late Transit 

 of Venus this has an important bearing, as the actinic intensity 

 of the sun diminishes from the centre to the limb, the highest 

 estimate given for it being from about 1 to y 2 T . The increase 

 in diameter of the sun and decrease in that of Venus will be 

 measurably the same. 



To diminish irradiation due to the first cause pointed out, three 

 methods are open : — first, to coat the back of the plate with 

 some non-actinic colour ; secondly, to increase the opacity of the 

 film ; thirdly, to connect the particles of bromo-iodide of silver 

 with a body of as nearly as possible the same density. The first 

 is the usual method adopted by photographers in dry-plate pro- 

 cesses. This diminishes the irradiation from the back of the 

 plate, but does not completely eliminate it, as will be understood 

 when the nature of the resulting reflection is considered. The 

 second method is easily carried out, and is more effective than 

 the first method. The last is a plan adopted in certain dry-plate 

 processes. With albumen preservative particularly, the amount 

 of irradiation is reduced materially, even though the film 

 appears nearly transparent. One other method may be noticed, 

 viz. staining the collodion itself with an adiactinic colour. This 

 does not seem to commend itself, as the sensitiveness of the plate 

 must be reduced in proportion to its efficacy. 



To diminish irradiation due to the second cause, Nos. 1, 2, 

 & 3 methods are effective. 



Since writing this paper I find that my friend Mr. Cowper 

 Ranyard has noticed, though I believe has not published, the 

 fact that irradiation is most marked at a distance corresponding 

 to the critical angle of reflection in glass. 



VIII. On an Apparatus to illustrate the Formation of Volcanic 

 Cones. By C. J. Woodward, B.Sc* 



A SHORT time ago I spent some weeks in Central France, 

 and had many opportunities of studying the character of 

 the volcanic cones which are so abundant in that district. The 

 many sections of these cones that may be seen at Graveneire 

 and other places, where the scoria? is obtained for industrial pur- 

 poses, show their structure clearly, and one can realize the 

 truth and simplicity of the theory accounting for their forma- 

 tion. The theory is so obviously correct that it has not, so far 

 as I know, been subjected to experiment on a small scale. The 

 object of this paper is to describe an apparatus that maybe used 

 in the lecture-room to build up model cones and craters. 



* Communicated by the Physical Society. 



