s - 1h e American Geologist. February L896 
the value of M for several positions of that lens. Hence it is 
convenient to make a small scale on the body of the tube by 
means of which it may be brought accurately into the same 
positions as when M was established as the standard of com- 
parison. M thus will have a value for each position; and 
when these values are once determined they can be applied t<> 
other minerals at any of those positions. 
When -1\\ is known the value of 2V can be calculated from 
the index of refraction by the equation 
l) sinE = n„, sinV, 
n,„ being the mean index of refraction of the crystal and n the 
index of refraction of the external medium. Hence 
n sinE. 
sillV " ,,,. 
Paris, Sept. 20, 1895. 
PHENOMENA OF FALLING METEORITES. 
By Oliveb C. Fabbington, Chicago, III. 
In the recent appreciative review made by Prof. N. H. 
Winchell of the Handbook and Catalogue of the Collection 
of Meteorites of the Field Columbian Museum, published by 
the writer, the reviewer propounds several queries relating to 
the question as to whether the so called " explosions ** usually 
accompanying the fall of a meteorite are really due to a 
bursting of its mass or whether the sounds may not be refer- 
red to other causes. As the points which Prof. Winchell 
raises well deserve consideration in any study of meteo- 
rites, the writer desires to set forth somewhat at length his 
views upon the subject, though conscious of his inability to 
discuss it exhaustively. It should first be stated, however, 
lhat " tin- theory of Proctor and others, that the chondri are 
due to aggregations of cosmic matter." was unknown to the 
writer at the time of publishing the Handbook, and he has 
-,, far been unable to find an account of it in Proctor's works. 
The only remarks which he has found concerning the struc- 
ture of meteorites are quotations of Sorby's views that the 
constituents of meteorites were originally in the state of 
vapor. A mention of these views was made in the Handbook. 
1. "Is not the idea that meteorites explode." asks the 
reviewer, " producing the detonations which accompany their 
fall, traditional and imaginary rather than actual?" 
