The fractions are determined as follows:  The fringes observed in the refractometer under the conditions above mentioned can readily be shown to be concentric circles.  The center has the minimum intensity when the difference in the distances, ab, ac, is an exact number of wave lengths.  The diameters of the consecutive circles vary as the square roots of the corresponding number of waves.  Therefore, if x is the fraction of a wave length to be determined, and y the diameter of the first dark ring, d being the diameter of the ring corresponding to one wave length, then x = y squared/d squared.

[Illustration: 

-----    +---+
|c     |   |
|      |   |
|      |   |+-------------------------+
|      |   ||                         |
|      |   ||                         |
|      |   |+-------------------------+ l
|      |   |            2.
|      |   | m
|      +---+
|                           ______
|            +-------------|      |---+
| /\    /\   | +-----------|    __|-+ |   _
a|/ /   / /   | |           | b |    | |  | |
S----------/\/__d/ /    | |           | | |    | |  | |
/ /|  / /-----| |-----------|-| ||||||||||| |
|  \/      | |           | | |    | |  | |
_|_         | |           |   |    | |  |_|
| :  |        | +-----------|___|----+ |  m
e| :  |         +-----------------------+
| :  |
|_ _|                  1. 
U
]

There is a slight difficulty to be noted in consequence of the fact that there are two series of waves in sodium light.  The result of this superposition of these is that as the difference of path increases, the interference becomes less distinct and finally disappears, reappears, and has a maximum of distinctness again, when the difference of path is an exact multiple of both wave lengths.  Thus there is an alternation of distinct interference fringes with uniform illumination.  If the length to be measured, the centimeter for instance, is such that the interference does not fall exactly at the maximum—­to one side by, say, one-tenth the distance between two maxima, there would be an error of one-twentieth of a wave length requiring an arithmetical correction.

Among other substances tried in the preliminary experiments were thallium, lithium, and hydrogen.  All of these gave interference up to fifty to one hundred thousand wave lengths, and could therefore all be used as checks on the determination with sodium.  It may be noted that in case of the red hydrogen line, the interference phenomena disappeared at about 15,000 wave lengths, and again at about 45,000 wave lengths; so that the red hydrogen line must be a double line with the components about one-sixtieth as distant as the sodium lines.—­Amer.  Jour.  Science.

* * * * *

[RURAL NEW YORKER]

COLD STORAGE FOR POTATOES.

Upon this subject I am able to speak with the freedom habitually enjoyed by some voluminous agricultural writers—­my imagination will not be hampered by my knowledge.