It is not possible at present to evaluate the accuracy of these estimates with the same precision as for other physical quantities, but the consistency of the readings from comparable plates of the same star will at least give a measure of the value of the estimates. Table XVIII contains the measures on forty-three lines in the spectrum of Gruis, taken from six plates of the same dispersion, and comparable quality, density, and definition. Successive columns give the wave-length, the arithmetic mean intensity, and the standard deviation . TABLEXVIII Line Int. Line Int. Line Int. Line Int. 4215 8.4 1.0 4319 4.8 0.8 4376 7.8 0.6 4451 4.5 1.5 4227 16.0 1.4 4321 4.0 0.0 4379 4.2 0.3 4455 5.0 1.0 4247 6.0 0.0 4326 10.4 0.5 4383 10.3 0.8 4462 6.0 0.8 4250 7.0 0.0 4330 4.6 0.8 4395 6.3 0.4 4482 7.7 0.7 4254 9.0 1.0 4332 3.6 0.5 4398 2.7 0.8 4490 7.3 0.4 4260 9.0 2.1 4333 4.0 0.0 4402 6.3 0.4 4495 7.5 0.4 4272 8.7 0.8 4337 8.7 0.8 4405 9.0 0.5 4502 6.0 0.0 4275 9.5 1.3 4340 9.5 1.1 4409 9.0 0.5 4554 5.3 0.4 4283 4.3 0.4 4352 9.2 1.1 4415 7.7 0.8 4564 5.8 0.7 4290 10.6 1.0 4360 6.8 1.0 4435 9.2 0.9 4572 6.0 1.0 4315 8.3 0.7 4370 6.8 0.6 4444 8.9 1.2 These measures are strictly representative of the material as a whole, for the plates of Gruis were measured at wide intervals in the ordinary course of the work, and were selected for illustration because there was a greater number of suitable plates of this star than for any other. HOMOGENEITY OF MATERIAL The observational material on line-intensities follows in tabular form. The measures were made in two groups, comprising respectively the hotter stars and the stars cooler than Class , and different intensity scales were used for the two. The solar scale mentioned above was used for the second group of stars; the first group was referred [Pg 119] to standard lines in the spectrum of Cygni. The distribution of the stars in the two groups among the spectral classes was as follows: Group I giants 4 7 7 6 8 6 3 17 super-giants - - - - 2 1 - - Group II dwarfs - - - - - 4 1 2 3 2 2 - - - - - - giants 1 9 3 5 2 - - - - 3 5 20 4 8 1 5 1 super-giants - - - - - 3 1 5 1 2 - 2? 1? 1? 3 1 -