408 Dr Pearson, On the Refraction observed [May 28, 



This new plane is represented in Plate X. with the usual Elba 

 combination. 



Gahnite (Zn0 4 (A1 2 )0 3 ). 



Three or more crystals twinned together according to different 

 {111} faces of the Tesseral system have long been known, and are 

 well described by Von Struver in Groth's Zeitschrift, Vol. II. 

 p. 480 in the case of Spinel. 



In the University collection there is a double twin consisting 

 of three perfect {111} faces each being equally developed, similar 

 to figure I. in Von Struver' s paper. I can find no record of such 

 a combination of crystals having been observed in Gahnite before, 

 and as I could not find one in any of the many collections I 

 examined, it may be considered a rare and interesting specimen. 



Von Struver in his work has considered the first crystal A as 

 the simple crystal and has then referred the other two crystals B 

 and C to it. This method gives to the {111} faces of C the high 

 indices of (11 11 1) and (13 5 7). 



But if we refer the two crystals A and C to B as the simple 

 crystal we get a simple symmetrical stereographic projection with 

 the {111} planes as {511}. 



I first observed such twins when working on the Zinc blendes 

 in the British Museum by perceiving one of the cubic planes in an 

 abnormal position, which would necessitate indices (744). 



That the face was that of the cube was proved by its physical 

 character, and it was necessary therefore to consider its abnormal 

 position to be due to double twinning about a central crystal 

 which in this case was so thin or so buried as to be invisible. 



Thus when the physical character of the faces is such as to 

 render it certain that they belong to a definite form, we may 

 suspect that they are faces on a portion of the individual which 

 owe their peculiar position to twinning about a central crystal, 

 and we are thus able in the case of blende to perceive the 

 existence of twinning even when the nucleus is so minute or so 

 completely enveloped as to be otherwise imperceptible. 



(5) On the Refraction observed at Sunset, (1) near Trinidad, 

 (2) near Rio. By Dr Pearson. 



1. On Jan. 2, 1883, I was in a ship anchored at Port-of- 

 Spain, Trinidad : Lat. 10 0- 39'. N. The horizon being quite clear 

 towards the west, I observed that the exact interval between the 

 first and second limbs of the Sun passing the horizon was two 

 minutes thirteen seconds. I had another equally favourable view 

 of sunset near Rio Janeiro, on March 28, in Lat. 18 0- 7'. S. when 



