54 Scientific Intelligence, 



any explanation of the fact that some of them are practically 

 complete in one direction. 



The qnalitatiye analysis is presented in the form of equations 

 of the reactions — a praiseworthy feature — but it seems that the 

 condensation of these tables is too great, and that too few details 

 of the operations are given to make the course a satisfactory 

 one for students. h. l. w. 



6. Neon Lamps for Strohoscopic Work. — "When a disk having 

 a pattern which is exactly duplicated in successive sectors is 

 revolved uniformly the pattern will coincide with, or so to speak 

 be superimposed upon, its initial position a certain number of 

 times each revolution. If the disk were to be illuminated only 

 at these instants the pattern would appear to be fixed in position. 

 In other words, if such a disk is illuminated n times per second 

 by very short flashes the disk will appear at rest whenever the 

 number of revolutions is an exact multiple or submultiple of n, 

 depending upon the number of times the pattern is repeated on 

 the disk. As the frequency of the flashes may be kept very con- 

 stant by the use of an electrically-driven tuning fork this princi- 

 ple affords the most accurate method of calibrating a revolution 

 counter. In practice the success of the method will depend upon 

 the duration and character of the intermittent illumination as 

 indicated in the following report of a study by F. W. Ashton 

 at the Cavendish Laboratory. 



The spark obtained by a Leyden jar discharge was not found 

 to be a satisfactory source of illumination as it is noisy in opera- 

 tion, feeble in intensity, and caused a rapid and excessive eye 

 strain. The author accordingly devised a form of neon filled 

 bulb to replace the condenser spark. The electrode spaces were 

 connected by a long capillary tube, coiled or doubled back on 

 itself several times, thus forming an analogue to the filament of 

 an ordinary incandescent lamp. The light comes from the posi- 

 tive column within the capillary and, as the brightness increases 

 with the current density, the reason for concentrating the dis- 

 charge within the tube is obvious.. A further advantage from 

 the length and coil of the capillary was that the discharge was 

 practically unidirectional and free from any travel of the 

 luminous effect. 



The color of the neon discharge is a brilliant orange-red con- 

 fined to the region 5700 to 6700 A. U. and is in such striking 

 contrast to daylight illumination that observations may be made 

 without darkening the room. The actual luminosity is perhaps 

 not so great as may be obtained from some other gases but the 

 fovea centralis is known to be extremely sensitive to red so that 

 its real effectiveness is greatly enhanced, and the experiments 

 showed that the eyestrain mentioned in connection with the Ley- 

 den jar spark was wholly absent. 



An investigation of the duration of the working flash was made 

 by observing the broadening of the filament when seen in a revolv- 



