Recent Advances in Electricity. 23 



The best determinations with which I am acquainted of the 

 length of Hertzian waves are those which Prof. Bose has made 

 by means of a diffraction grating. Hertz's own determinations 

 were chiefly made by means of resonators^ that is to say, by 

 causing the waves to excite oscillating currents in a conductor, 

 and choosing this conductor of such dimensions as would give 

 the strongest oscillations. This method has been found so 

 uncertain in its results as to excite a suspicion that the waves 

 have not in each case a definite wave length. We know that 

 the waves of light from incandescent sodium vapour have a 

 definite length, but that those emitted by incandescent iron or 

 charcoal have all possible lengths lying between certain limits. 

 The spectroscope, when applied to light from such a source, 

 shows a continuous spectrum, whereas when applied to sodium 

 light it shows the well-known sodium line. It is conceivable 

 that Hertzian rays have a continuous spectrum, and Bose 

 wanted to test this point. The spectrum of a beam of light 

 can be obtained either by means of prisms or by employing 

 a diffraction grating, which usually consists of a plate of 

 speculum metal, on which, by means of a dividing engine, a 

 number of equidistant scratches are ruled with a diamond 

 point, so close together that there are sometimes ten thousand 

 or twenty thousand to the inch. Bose imitated this arrange- 

 ment on an enlarged scale, suitable to the different magnitude 

 of the length to be measured. Instead of scratches some 

 thousands to the inch, he used metallic strips an inch or so 

 apart, arranged so as to form a concave cylindrical grating, 

 analogous to the concave gratings employed by Rowland for 

 light. No slit was necessary, as the line of discharge of the 

 sparks was' sufficiently narrow of itself, and was kept parallel 

 to the strips of the grating. A cylindrical grating focuses the 

 spectrum without the aid of a lens, and when the distances 

 are properly adjusted the various colours are brought to a focus 

 at successive points on the circumference of a certain circle. 

 Professor Bose made his observations by moving his spiral- 

 spring receiver slowly along this circle, and he found that the 



