370 



SUMMAEY OF QUERENT RESEARCHES RELATING TO 



it is easy to cause the line of demarcation between the two fields to dis- 

 appear. The method of using the photometer is shown in fig. 62, in 

 which Si and S2 are the two sources whose intensities are to be com- 



FiG. 62. 



pared, and P the photometer. In addition to its simplicity and com- 

 pactness, the other advantage claimed for this photometer is that it 

 can be used under all conditions of angle which the two beams whose 

 intensities are to be compared make with one another 

 (with the exceptions, of course, of absolute normal and 

 grazing incidence). A photometer of this kind has 

 already been used in a determination of the distribu- 

 tion of light in the various spectra of a grating,* and 

 has yielded very excellent results. 



Swift's Pan-aplanatic Low Power Condenser.f 

 This condenser (fig. G:-}), on the triple posterior system, 

 is intended for use with medium powers whose N.A. 

 does not exceed 0-60. It has a N.A. of o-5, and is so corrected 

 that the aplanatic aperture is within a trifle of its N.A. 



(4) Photomicrography. 



Selection of Plates and Filters for Photomicrography. | — The firm 

 of Wratten and Wainwright have issued a table of notes and instructions 

 for the use of their plates and screens, which will, they think, enable 

 the photomicrographer to obtain records of objects which he has long 

 given up in despair. 



* Astrophysical Journal, xxi. No. 2 (1905). 



t Swift and Son's Special Catalogue, 1906, p. 50, fig. 54. 



X Catalogue published by Wratten and Wainwright, Croydon (1907) 13 pp. 



