NEBULA, CLUSTERS, ETC. 



49 



Philosophical Transactions: Royal Society, London. 



Philosophical Transactions : Button's Abridgment to 1800. 



Pickering: On a nebula photometer. Am. Jour. Sci., 3d series, xi, 1876, 

 June. 



: Physical Manipulation, 1876. 8°. ii, p. 307. [Experimental 



method for measuring the brightness of nebulas.] 



Pigott : Account of a nebula in Coma Berenices. P. T., 1781, p. 82, and 

 p. t., xv, p. 37. 



Pihl : Micrometric examination of the stellar cluster in Perseus. Christi- 

 ania. 1869. [Has a chart.] 4°. See V. J. S., 1870, p. 133. 



; On the cluster in Perseus. [With a chart.] Mon. Not. R. A. S., 



xxviii, p. 247, and xxix, p. 329. 



Plante: C. R., lxxxi, p. 749. [Possible explanation of the formation of 

 spiral nebula?.] 



