NATURE 



[Nov. 6, 1879 



magnified by the full power of the telescope. If the two 

 images are separated by a distance d, any two stars at about 

 this interval maybe brought together, so that any star may be 

 compared with all those on the circumference of a circle having 

 a radius d. With a large prism in which d equalled about 3 , 

 an attempt was made to compare /3 and p Persei and and 7 

 Lyre by placing the prism in front of the objective of a 

 telescope having an aperture of about 10 cms. A direct 

 measure of the variations in brightness of the above-named 

 variable stars might thus be obtained. This plan was abandoned, 

 owing to the colour of the images. 



" There is one other position of the prism, that where the eye- 

 piece forms its image of the abjective, in which the emergent 

 pencil will remain undivided. This is, however, the exact 

 point at which the eye should be placed ; and, moreover, the 



interval between the images cannot, in this ca=e, be altered. 

 Good results were obtained by placing the prism a little nearer 

 the eye-piece, as in the first of the instruments described below. 

 The advantage of placing the prism between the eye-lens and 

 field-lens is that it is less likely to reduce the field of view. As 

 this plan is open to the double objection of dividing the emer- 

 gent pencil and keeping the images always at the same distance 

 apart, it has not been employed in the following observations. 



"The fourth position for the prism is between the field-lens 

 and objective. It then separates the emergent pencils by an 

 amount increasing with its distance from the objective, but, on 

 the other hand, the interval between the images is proportional 

 to its distance from the focus. Whatever, therefore, is the 

 interval between the two stars, within certain limits, their images 

 may always be made to coincide by first [turning the prism and 



A *,'\\7 if. 



then sliding it along the axis of the telescope to the proper 

 distance from the focus. A prism may therefore be used, in 

 which the separation is small, and thus the two images of the 

 objective may be rendered nearly coincident. 



"The position of Nicol is comparatively unimportant. 

 Since it must turn without moving the double-image prism, it is 

 more convenient to place it between the latter and the eye. It 

 was, accordingly, sometimes placed between the eye-lens 

 and the eye, and sometimes between the field-lens and eye-lens." 



We now come to the instruments. 



The first observations were made with an eye-piece having a 

 Nicol between it; two lenses and with the double-image prism 

 between this eye-lens and the eye. The observations made with 

 this apparatus are regarded as preliminary ; a second photometer 

 was constructed, in which the Nicol and double-image prism 



were both placed in front of the eye-lens, the Nicol being next 

 the eye. One marked advantage of this instrument, was that 

 the circle instead of the index, turned .with the Nicol. The 

 labour of reading was thus much reduced. The Nicol was also 

 replaced by a double-image prism, with the advantage that 

 the field of view was less obstructed. With this form, however, 

 the great number of images formed by successive reflections, 

 when a bright object was observed, rendered it sometimes 

 difficult to determine which should be compared. 



A much simpler arrangement was used later. It consisted 

 of two concentric tubes, one carrying a graduated circle, the 

 other two indices. In the first of these tubes, a double-image 

 prism was inserted ; the other, which was held next the eye, 

 carried a Nicol. This photometer was used without a telescope 



Fig. 3 (Scale I). 



to compare the relative brightness of Saturn and Mars, and 

 Jupiter and Venus. A tube was attached to this photometer, 

 so that the light should always pass nearly normally through the 

 prisms. When the objects were sufficiently bright, and within a 

 few degrees of one another, good results were thus obtained, 

 but the colour of the imager, and their want of symmetry, was 

 a serious objection when a great difference in light was to be 

 measured. 



After an experience of some months with these instruments, 

 certain improvements suggested themselves, and still another 

 photometer was constructed, represented in perspective in 

 Fig. I, and in section, on a scale of one-fifth, in Fig. 2. In 

 both figures, B represents the eye-piece, in front of which is 



Fig. 4 (Sea le !). 



inserted a N icol, A. A circle, divided into degrees, is attached, 

 and turns with the eye-piece. The indices D D are fastened to 

 the tube E, which slides into the telescopes. F is a rochon 

 prism, which was used instead of a double-image prism of spar. 

 As it consisted of quartz, the separation of the images 

 amounted to somewhat less than 1°, so that the emergent pencils 

 overlapped each other by nearly three quarters of the diameter 

 of each. The apparatus had, m ireover, the great advantage 

 that the images were precisely alike and nearly achromatic. 

 The prism was placed in a tube, which could be drawn towards 

 or from the eye-piece by a cord G. Attaching this photometer 

 to a telescope, and directing it towards a star, the latter appeared 

 double ; and the interval between the components might be 

 altered at will. 



