OF ARTS AND SCIENCES. S99 



power, or by its three heavy and burred lines when seen under higher 

 power. Its position near tlie brightest part of the spectrum, together 

 with the readiness with which it can be recognized, make this group 

 one of more than common interest. Moreover, since it combines, 

 witliin very narrow limits, lines of almost every grade of heaviness and 

 character, it would seem to have every qualification for a " test group " 

 upon which the performance of any spectroscope may be tried. In- 

 deed, for such a purpose it is equalled by few, and surpassed by no 

 other groups of the spectrum. 



The following is a brief description of the instruments employed : — 

 The spectroscope is mounted on a four-legged table, and the light is 

 received through a circular aperture 11 centim. in diameter, from a 

 " porte-lumiere " fixed in a window upon the southern face of the 

 building. The mirror of the porte-lumiere is of silvered glass, and 

 measures 10 centim. by 2G centim., and in this position can reflect the 

 sunlight for about four hours only, during the day ; that is, from 9 a.m. 

 to 1 P.M. Before falling upon the slit of the collimating telescope, 

 the light passes through a small cylindrical condensing lens measuring 

 4 centim. each way, with a focal length of 25.6 centim. This con- 

 denser is held in front of the slit by means of a wooden clamp. The 

 collimator has an aperture of 4.76 centim., and focal length of 50 

 centim. The slit is 2.1 centim. long, and its width is adjustable by 

 a rather coarse-threaded screw. The observing telescope is of 4.7 cen- 

 tim. aperture, and 49.6 centim. focal length. Three eyepieces can 

 be used, and have been designated 1, 2, and 3. Their magnifying 

 powers are appi'oximately 35, 25, and 18 diameters. Both observing 

 telescope and collimator admit of motion about two centres, so that 

 proper adjustment can be made for any part of a grating, or for any 

 number of prisms. 



It was found that, owing to unsteadiness in the instrument, no 

 accurate measurements could be made with the filar micrometer ; so, 

 abandoning the micrometer, a finely divided scale, with which all the 

 measures were made, was placed in the highest power eyepiece. This 

 method of measurement appears to possess all the accuracy attainable 

 under the circumstances. The "eyepiece scale" was ruled by Pro- 

 fessor "W. A. Rogers upon a piece of glass 0.5 mm. in thickness, and 

 19 mm. diameter, cemented to a diaphragm at the focus of the eye- 

 piece. The lines of the scale are seen with great distinctness, and, 

 owing to the thinness of the glass upon which they are ruled, no appre- 

 ciable amount of light is lost. Through the kindness of Professor 

 Rogers I have been enabled to make a careful investiiration of the 



