A table of the atomic refractions and **dispersions** of the principal elements is here given: Dispersion and Composition.-In the preceding section we have seen that substances possess a definite molecular (or atomic) refraction for light of particular wave-length; the difference between the refractions for any two rays is known as the molecular (or atomic) dispersion.

Since molecular refractions are independent of temperature and of the state of aggregation, it follows that molecular **dispersions** must be also independent of these conditions; and hence quantitative measurements should give an indication as to the chemical composition of substances.

He also showed how changes in constitution effected **dispersions** to a far greater extent than they did refractions; thus, while the atomic dispersion of carbon is 0.039, the **dispersions** due to a double and treble linkage is 0.23 and 0.19 respectively.

The figures given are the partial **dispersions** for ordinary crown and ordinary extra dense flint glasses, styled in Messrs Schott's catalogue of optical glasses as o 60 and 0.102 respectively, having refractive indices of 1 5179 and 1.6489 for the D ray respectively, and (µ D -I)/(l F -µc) =60 2 and 33.8 respectively to indicate their dispersive powers (inverted), = v.

The table gives their partial **dispersions** for six different regions of the spectrum also expressed (in brackets below) as fractional parts of the dispersion from C to F.

Since the curvature powers of the positive lenses are equal, the partial **dispersions** of the two glasses may be simply added together, and we then have: [0.543 +0.3741 The proportions given on the lower line may now be compared with the corresponding proportional **dispersions** for borosilicate flint glass 0.658, closely resembling the type 0.164 of Schott's list, viz.: [0.658 (A D = I.546) 50' 11 A slight increase in the relative power of the first lens of 0.543 would bring about a still closer correspondence in the rationality, but with the curves required to produce an object-glass of this type of 6 in.