PHYSICS: C. BARUS 
39 
integration. Further than this, a meteoric stone would be less likely to 
attract the attention and curiosity of the ordinary individual than would an 
iron. So far as the first possibility is concerned, I think that all who have had 
to do with meteorite collections will agree that as a general rule the irons, 
through their susceptibility to a damp atmosphere and consequent rusting, 
require much more attention for their preservation than do the stones. The 
second possibility is, however, one that must be given consideration. 
1 See Chapter IV of Farrington's Meteorites, Chicago, 1915. 
2 The figures here given relative to number of falls are believed to be substantially cor- 
rect up to 1916. Accurate statistics since that date are not available. 
3 It would be a natural supposition that the fall of an iron would be less noticeable than 
that of a stone since the former would be less liable to break up — explode — in its passage 
through the atmosphere. Unfortunately, the literature is not sufficiently explicit on this 
point to bear out the supposition. Hidden, to be sure, states that the fall of the Mazapil 
iron was accompanied only by a loud sizzing sound, there being no explosion or loud deto- 
nation. On the other hand, Kunz states that the fall of the Cabin Creek iron was "ac- 
companied by a very loud report which caused the dishes to rattle," and the fall of the 
NedagoUa iron is also stated to have been accompanied by an explosion. Accounts of 
other falls are either noncommittal on this point or equally contradictory, and it is evident 
-accurate information is lacking. 
NOTE ON A CONTACT LEVER, USING ACHROMATIC 
DISPLACEMENT FINGERS 
By Carl Barus 
Communicated, December 27, 1918 
I. Apparatus. — The method heretofore described for the measurement of 
small angles by the aid of the rectangular interferometer, lends itself con- 
veniently for the construction of apparatus like the contact lever, or the 
spherometer. Having work needing such instruments in view, I designed the 
following simple apparatus for the purpose. 
Figure 1 is a plan of the design; figure 2 an elevation of the fork and appur- 
tenances; figure 3 (plan) finally shows the same apparatus adapted for use 
as a spherometer. The interferometer receives the white light from a colli- 
mator at L. After the reflections and transmissions controlled by the mirrors 
M, M\ N, N' , and the auxiliary mirror mm' , as indicated in the figure, the 
light is conveyed into the telescope at T for observation of the interferences. 
The mirror M' , is on a micrometer with the screw s normal to its face. 
It is through the mirror mm' that the small angles are to be measured and 
this is therefore mounted at one end of the lever dc, capable of rotating around 
the long vertical axle aa, in the circular fork FF. The latter is rigidly mounted 
on the bed of the apparatus by aid of the stem / in the rear. The lever c is 
bent upward at right angles at and it is here that the mirror mm' is firmly 
