1904.] On the Temperature Classification of Stars. 



231 



4. Description of the Instrument used. 



With regard to the new apparatus, I may state that an objective 

 prism camera (fig. 1), was devised having a 2-inch 30° calcite prism 

 mounted in front of a 2j-inch quartz lens of 18 inches focal length. 



SSI 



Fig-. 1. — Quavtz- Calcite Prismatic Camera. 



The prism is so cut that its first face is perpendicular to the optic 

 axis of the crystal, and it is arranged that the incident rays are normal 

 to this face. All the rays, therefore, pass through the prism parallel 

 to the optic axis and in this way there is no double refraction. 



The whole length of the spectrum is brought into focus in a plane 

 inclined at 42° to the optical axis. The apparatus is attached to the 

 side of an equatorially mounted 6-inch Dallmeyer doublet camera with 

 an angle in declination between the two optical axes equal to the angle 

 of deviation of the calcite prism, so that the Dallmeyer is used as a 

 finder. 



Edward's snap-shot isochromatic plates have alone been used, and 

 as they are not sufficiently sensitive for wave-lengths between A, 486 

 and A 550 (approx.), there is a break in each spectrum about this 

 region followed by a further portion of the spectrum having its centre 

 about "D." The length of spectrum obtained is such that to 

 H e is 0-355 inch (9*0 mm.) and from H e to H y 0-165 inch (4'2 mm.). 



5. Method of Work. 



As shown in Table I, there are ten stages or steps of temperature to 

 be investigated, and pairs of stars were selected in order to show the 

 differences between adjacent stages or else the difference between 

 stages widely apart. 



Stages 1 and 10 are not available in consequence of the photographic 

 faintness of the Antarian and Piscian stars, and the fact that the only 



