Mr. Sylvester on the Optical Theory of Crystals, 537 



exposed to the influence of the sun under water, the disengage- 

 ment of the oxygen from the divided air-ducts soon ceased ; it 

 began however anew if the leaves were again placed in their 

 natural position. 



Dutrochet finally infers from his various experiments that 

 plants at night absorb the oxygen from the air, and that this 

 is only an auxiliary respiration, while the true process of re- 

 spiration of vegetables consists in the disengagement and dif- 

 fusion of the oxygen in the interior of the plant caused by the 

 solar light. 



Morren*, who made several experiments in the botanical 

 garden at Louvaine on the respiration of plants, observed on 

 the 18th of May, during the great solar eclipse, that the re- 

 spiration of the green parts of plants, that is the expiration of 

 oxygen, entirely ceased at that time. We can observe some- 

 thing very similar to this on very warm summer days, when 

 for instance the respiration of oxygen is very considerable 

 from the action of solar light, and the sun all at once disap- 

 pears under dark clouds ; I have noticed several times how 

 soon the disengagement of gas bubbles diminished, and at 

 last ceased more or less completely. 



[To be continued.] 



LXXVI. Analytical Development of FresncVs Optical Theory 

 of Crystals. By J.J. Sylvester, Member of St. John's 

 College, Cambridge. 



[Continued from p. 469.] 

 Cor. — Hence we may reduce the discovery of the two 

 fronts into which a plane front is refracted on entering a 

 crystal to the following trigonometrical problem, 

 p 



Let a sphere be described 

 about any point in the line in 

 which the air front intersects 

 the plane of incedence. Let 

 the great e P I denote the 

 latter plane, I F the former, 

 O A, O C also great circles, 

 the planes of single velocity. 

 Suppose I G H to be one of 

 the refracted fronts inter- 

 secting O A, O C in G and H, then 



( a* + c g ) - (a*-&') cos( G + H) _ sin ( P I F)* 



2 (vel. in air)* = (sinPIGH) 2 ' 



* Vlnstitut 18S6, p. 416. 



Third Series. Vol. 11. No. 70. Dec. 1837. 3Z 



