388 Scientific Intelligence. [Nov. 



images separated 34', just double of that which is effected by either of the preced- 

 iug combinations. 



Though it could scarcely be doubted that this is essentially the construction 

 which was employed by M. Rochon, there is an additional circumstance concern- 

 ing the effect of such a pair of wedges uhen otherwise combined, which fully esta- 

 blishes the identity of the method here proposed with his. If the two wedges be 

 placed with their edges together, so as to form by their union a wedge of 40°, the 

 consequence is, that though a pencil of light is in fact divided into two parts by 

 the first wedge, both parts in the end emerge together; the refraction of one being 

 o + e, and of the other e + o: they both deviate from their original direction by 

 exactly the >ame quantity, and present only a single image of the luminous object; 

 but it is coloured, as usual, in proportion to the amount of deviation occasioned 

 by the sum of the wedges. This, without doubt, is the first of two opposite direc- 

 tions mentioned by M. Rochon, in which he says the double refraction was not 

 perceptible. 



" Pour cet effet," says M. Rochon, " j'employai deux prismes ^aux tallies dans 

 lesens le plus favorable ames vues, etenles presentant dans les deux sens opposes je 

 trouvai, que dans la premiere disposition la double refraction n'etoit pas percep- 

 tible, niais, en faisant prendre a mes prismes un sens inverse, la double refraction 

 de chaque prisme eloit presque doublee." 



The correspondence in the effect which I have described renders this passage from 

 M, Rochon perfectly intelligible; and I hope the directions above given will be 

 sufficient to enable any one to cut a crystal to the greatest advantage for making 

 this sort of micrometer. But it must be observed that in attempting such a con- 

 struction, great nicety is requisite not only in cutting the wedges so that the refrac- 

 tion in each shall take place at right angles to the axis, but also in cementing theui 

 together, so that the axes of the two wedges shall be at right angles to each other. 

 And it may farther be remarked, that even then, unless the pencilof light pass truly 

 in the common plane of refraction of the wedges, four images will be formed, so 

 as to destroy the effect of the combination. 



The part terminates as usual with a meteorological journal of 

 the weather at London during the year 1819. 



The mean height of the thermometer was 52"7°; of the baro- 

 meter (not corrected for temperature) 29'81. The quantity of 

 rain was 13'727 inches. The mean of Leslie's hygrometer was 

 16-8°. 



Article XII. 



SCIENTIFIC INTELLIGENCE, AND NOTICES OF SUBJECTS 

 CONNECTED WITH SCIENCE. 



I. Remarkable Petrifaction. 



The city of Glasgow is built upon the coal formation which 

 extends round it in every direction. The sandstone beds 

 belonging to this formation are in many places, just at the 

 surface of the earth, of great thickness, and almost every where 

 constitute excellent building stones. The quarries of this sand- 

 stone, out of which the city has been built, are situated chiefly on 

 the north-west side of the town below port Dundas. These 

 quarries are very extensive. The uppermost beds of sandstone 

 are usually of a reddish hue, and some of them abound in iron 



