Chemistry and Physics. 61 



tus, Zenger was struck with the great increase in accuracy ob- 

 tained by illuminating the slit with monochromatic light ; and 

 this led to the employment of a direct-vision pocket spectroscope, 

 the collimating lens of which was removed and the slit replaced 

 by a silvered cylindrical mirror, 16mm. in diameter. The whole 

 was mounted upon a board, the mirror sliding in a groove so that 

 its distance from the spectroscope could be readily adjusted. 

 The edge of this groove was divided into millimeters. Standing 

 with one's back to a window and looking through the spectroscope 

 at the narrow line of light reflected from the mirror, a brilliant 

 spectrum is seen, the Fraunhofer lines appearing at the eye end 

 of the instrument. But in order that the different lines should be 

 in focus, it is necessary to adjust for each by moving the mirror 

 to or from the prism, the distance of a single millimeter causing 

 these lines to appear or disappear. Hence the distance of distinct 

 vision, for normal eyes and for a given line may be readily deter- 

 mined ; these distances showing a difference of one to one and a half 

 centimeters in passing from the lines D or E to the lines F or G. 

 For abnormal eyes, the finer lines may be used and the degree of 

 sensitiveness for different colors determined with great sharpness, 

 agreeing to within a millimeter. In practice the method is found 

 satisfactory. — C. JR., ci, 1003, Nov. 1885. G. f. b. 



7. On Magnetization by the Electric Discharge. — Many years 

 ago Savary showed that when a steel needle was magnetized by 

 means of an electric discharge in its vicinity, the amount of the 

 effect produced was variable and the direction not uniform. 

 Claverie has confirmed these results and has given a satisfac- 

 tory explanation of them. His experiments were made with a 

 battery of 12 jars each of the capacity of 0*01 microfarad, 

 charged to a potential determined by the length of spark as 

 measured with a micrometer. The needles, previously hardened, 

 were placed in a magnetizing spiral 300 mm. long and 13 mm. in 

 diameter, made of a copper wire 0*5 mm. in diameter, the turns 

 being ^ of a millimeter apart. In this spiral, the magnetic field 

 produced by a unit current was practically uniform for a needle 

 15 mm. long, varying only irom 97TX 1*998 in the center, to 

 9;rx 1*9975 at a distance of 5 centimeters in either direction. 

 The author finds that such a needle 0*5 mm. diameter, magnetized 

 in this spiral by an electric discharge whose striking distance is 

 10 mm., develops on immersion in hydrochloric acid, alternate 

 polarities as the layers are successively dissolved. On examining 

 the needle at intervals of ten minutes its magnetic moment, in 

 arbitrary units, was found to vary as follows: +20*5, + 14*5, 

 + 5, —5, —11, —15, —18-5, —18, —13-5, —16, —13, —9, —6, 

 -3-5, -2, +5, +7, +8, +10-5, +12, + 11*5, +11, + 8*5, + 6*5, 

 + 6, +3, +2, +1, —0-5, —0-5, +0.5. The plus sign here in- 

 dicates polarity according to Ampere's law. Now, Feddersen 

 has shown that the discharge of a condenser, through a circuit of 

 low resistance, consists of oscillations between the armatures, 

 with a gradually decreasing intensity ; and that as the external 



