216 A. A. Michelson — Interference Experiment. 



It will be seen that the two analyses agree throughout and 

 the formula, is the same, viz : R 3 P 2 8 +^H a O. As noted 

 above it is identical with that of dickinsonite, although 

 the latter species contains more alkalies and less manganese. 

 The two species are then essentially dimorphous forms of the 

 same compound, and the relation between them is made all the 

 more interesting in that with the striking differences in 

 physical characters, there is yet an obvious relation in form. 

 Dickinsonite is monoclinic with marked pseudo-rhombohedral 

 symmetry and of h'llowite the same is true as we have proved 

 by a reexamination of fragments parallel to the distinct but 

 interrupted basal cleavage. Moreover the dimensions of the 

 forms show a close relation, thus we have : 





Dickinsonite. 



FilJowite. 



100^001 = 



61° 30' 



58° 31' 



001 ^ 221 = 



61 8 



58 40 



001^221 = 



61 8 



58 40 



"We have then in these two species an example of a very close 

 and interesting case of dimorphism. The suggestion that the 

 two could be regarded as independent forms of the same 

 mineral differing in habit and state of aggregation could not 

 possibly be made by one who had seen and examined the 

 specimens. We have still hope that in future explorations at 

 Branchyille we may find a new supply of this rare and inter- 

 esting species, named in honor of our good friend, Mr. A. !N". 

 Fillow. 



Art. XXYII. — A simple Interference Experiment; by 

 Albert A. Michelson. 



There is probably no experiment connected with the wave- 

 theory of light of greater fundamental importance than the 

 justly celebrated one known as " Fresnel's mirror experiment," 

 and accordingly many find it necessary to repeat the experi- 

 ment for their own edification or for class demonstration. 



Without the use of rather elaborate and costly apparatus 

 even skilled experimenters find some difficulty in producing 

 the desired effect — the interference of two pencils of light as 

 manifested by the appearance of colored fringes on a screen, 

 or in the focus of an observing lens ; and unskilled observers 

 find it almost impossible, even with these aids. Even when 

 produced under the most favorable conditions the phenomenon 

 is complicated and often much obscured by diffraction effects. 



