the Refractive Index and Dispersion of Glass. 411 



of n D is of the order o£ one in the third place of decimals 

 rather than one in the fourth. 



The method here described was devised with a view to 

 obtaining a direct measurement of the refractive index of 

 the glass, thus obviating the necessity for making intermediate 

 observations which, by their unreliable nature, greatly limit 

 the accuracy finally obtainable. Tiie rationale of the method 

 consists in immersing the glass under examination in a 

 liquid whose refractive index for the specified wave-length 

 can be varied continuously until an equality is attained 

 between the index of the glass and that of the surrounding 

 liquid. The ordinary process can then be applied to measure 

 the refractive index of the liquid on the Pultrich refracto- 

 meter. It should be noted that the method may be employed 

 for the refractometric examination of glass in any irregular 

 shape, and not solely of glass in lenticular or prismatic form. 

 It is not claimed that the method is an absolute one for the 

 determination of the refractive index of the glass, as the 

 ultimate accuracy is, of course, that set by the measure- 

 ments on the refractometer; but it is possible with care to 

 equalize the refractive indices and partial dispersions of the 

 glass and the immersion fluid to at least as high an order 

 of accuracy as that with which they can subsequently be 

 measured on the refractometer. 



The "Schlieren-methode" invented by Topler and de- 

 scribed in detail in Wied. Ann. vol. cxxxi. p. 33 (see also 

 R. W. Wood, 'Physical Optics,' p. 78) affords an extremely 

 sensitive means of detecting the presence in an otherwise 

 homogeneous transparent medium of stria? or other optical 

 non-homogeneity, and is therefore well suited to the present 

 purpose. The arrangement of the apparatus employed is 

 shown diagrammatically in plan in fig. 1. 



Fig. ] . 



A vertical straight edge behind which is placed a source 

 of monochromatic light A is represented in plan by B. An 

 image B' of this straight edge is projected bv means of a 

 telescope objective C (focal length 125 cm., aperture 8 cm.) 

 into the plane of a second straight edge, F, disposed as 

 shown in front of an observing telescope H, so as to cover 

 about one-half of the full aperture of its objective G. The 



