.">7i> Scientific Intelligence. 



The paper under review concludes with details of the optical 

 peculiarities of some highly interesting exceptional cases. Suffice 

 it to say that these exceptions are afforded by the blue-winged 

 butterfly, Morpho alga, by the "Diamond Beetle," and by the 

 coleopter Plustiotis re^)lendens. — Phil. Mag. (6), xxi, 554. 



II. s. u. 



6. The Isolation of an Ion, a Precision Measurement of its 

 Charge, and the Correction of Stokes's Law. — In the earlier work 

 of R. A. Millika.it the following sources of error introduced 

 some uncertainty in the final results. (1) Lack of complete stag- 

 nancy in the air through which the drop moved ; (2) lack of per- 

 fect uniformity in the electrical field used ; (3) the gradual 

 evaporation of the drops, rendering it impossible to hold a given 

 drop under observation for more than a minute, or to time the 

 drop as it fell under gravity alone through a period of more than 

 five or six seconds ; (4) the assumption of the exact validity of 

 Stokes's law for the drops, studied. The author says: "The 

 present modification of the method is not only entirely free from 

 all of these limitations, but it constitutes an entirely new way of 

 studying ionization and one which seems to be capable of yield- 

 ing important results in a considerable number of directions." 



The essential modification of the method consists in replacing 

 the droplet of water or alcohol by one of oil, mercury, or some 

 other non-volatile substance, and in introducing it into the ioniza- 

 tion chamber in a new way. By means of a commercial " atom- 

 izer " a cloud of fine droplets of oil is blown with the aid of 

 dust-free air into a vertical, dust-free cylinder. The base of this 

 chamber is made of a heav} r , circular disc of brass which consti- 

 tutes the upper plate of the electrical condenser and which is per- 

 forated through its center by a vertical hole having about the 

 same diameter as an ordinary pin. This hole is provided with 

 a trapdoor or cover which is actuated electro-magnetically. 

 This trap is kept open long enough to allow a few drops to fall 

 through it into the condenser below and then the cover is closed. 

 In this way it is possible to imprison droplets in a condenser con- 

 taining dust-free air which is absolutely free from currents. The 

 condenser plates were exactly 10 ram apart. A narrow, horizontal, 

 parallel beam of light from an arc lamp entered the condenser 

 through one window, illuminated the droplets in its path, and 

 then passed out through a diametrically opposite window. The 

 illuminated drops were observed through a third window by 

 means of a suitable telescope of short focus. The appearance of 

 a drop is that of a brilliant star on a black background. The 

 drop falls, of course, under the action of gravity, toward the 

 lower condenser plate ; but before it reaches the latter an electri- 

 cal field of strength between 3,000 and 8,000 volts per centimeter 

 is established between the plates by means of a battery. Hence, 

 if the droplet had received a frictional charge of the proper sign 

 and strength as it was blown out through the atomizer, it will be 

 pulled up against gravity by this field, toward the upper condenser 



