Notices respecting New Books, 



127 



or a half vibration-length. Thus by similar triangles the length 

 M N is divided into the same number of equal parts as A B. 

 This being the case, any length such asNP can be at once com- 

 pared with a length N M, and consequently the time value of 

 the length KP. I find that for accuracy, the method compares 

 very favourably with the micrometer method, while the advan- 

 tage in time of working is as about four to one. When a large 

 number of determinations have to be reduced the question of 

 time is important. If a well-made aluminium stylus draws a 



trace on a smoked-glass surface, the trace when examined by 

 a microscope appears to have a bright line situated midway in 

 the cutting excavated by the stylus, this line is always used 

 to measure from ; the edge of the trace is of no value, as its 

 outline is usually rather like the teeth of a mason's saw. For 

 many determinations, such as those of wave-motions in solids, 

 it has been found convenient to use a fork having a period of 

 ^ second ; by using the wedge-divider, the —^ second may 

 be at once read. The surface velocity of the glass is arranged 

 so that each vibration is about 0*8 centim. long. My first 

 experiment, which led up to the wedge-divider used in con- 

 junction with a microscope, was to put a trace into a projec- 

 tion lantern, and measure the image on a cardboard screen 

 carrying a triangle, divided as I have described. By using 

 the wedge-divider in close contact with the trace, in the 

 field of a microscope, the distorsion inherent in the lantern 

 method is eliminated. 

 Oxford, June 12, 1891. 



XX. Notices respecting New Books. 



Telescopic Work for Starlight Evenings. By William F. Denying, 

 F.R.A.S. (Taylor and Francis.) 



"TNIYEESALLT recognized as one of the greatest living author- 

 ^ ities on the subject of Meteoric Astronomy, Mr. Denning 

 has also discovered two comets, and made numerous and accurate 



