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XXXVI. A neio Arrangement for the Micrometer of the Auto- 

 matic Spectroscope. By Walter Baily *. 



IN some spectroscopes the micrometer-wires are seen by 

 means of a pencil of light reflected from the surface of the 

 last prism ; but this arrangement is not applicable to the auto- 

 matic spectroscope, because the last surface of its last prism is 

 perpendicular to the axis of the eye-telescope. An equivalent 

 arrangement may, however, be made by using one of the forms 

 of prism whose sections are given in the figures, as the last 

 prism of an automatic spectroscope. The light to be analyzed 

 having passed through the rest of the train of prisms, enters 

 the last prism, which is immovable, through the side A B, and 

 emerges through C D, the light which is brought to a focus in 

 the centre of the field emerging perpendicularly to C D. Let 

 the light from the cross-wires be made parallel by a lens, and 

 let P Q be the direction of the light from the centre of the 

 cross. In fig. 1 P Q is perpendicular to the axis of the eye- 

 telescope, in fig. 2 P Q is inclined at an angle of 120°, and in 

 fig. 3 at an angle of 60° to this direction. P Q R S gives the 

 course of the light from the centre of the micrometer-cross ; 

 and R S is perpendicular to the side C D ; and therefore the 

 centre of the cross will be distinctly seen in the centre of the 

 field at the same time as the spectrum. The form of prism to 

 be used depends upon the position in which it is desired to 

 place the micrometer. The diameter of the pencil of light 

 from the micrometer is taken in the figures as one third of that 

 through the prisms ; and the latter is taken as the unit of length 

 in the following dimensions for each prism. 



Fig. 1. Fig. 2. 



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* Communicated by the Author. 



