136 Royal Society : — Dr. W. Huggins on the 



mator is so constructed that, by means of a coupling-screw, any one of 

 three spectroscopes can be conveniently attached to it. 



This apparatus performs admirably ; but it seemed to me desirable, 

 for observations of great delicacy, to be able to dispense with reflec- 

 tion, and to place the source of the light for comparison directly before 

 the slit. Formerly I accomplished this object by placing the spark 

 or vacuum-tube before the object-glass of the telescope. The great 

 length of the present telescope renders this method inconvenient; 

 but a more important objection arises from the great diminution 

 of the light when the spark is removed to a distance of 1 5 feet from 

 the slit. I therefore resolved to place the spark or vacuum-tube 

 within the telescope at a moderate distance from the slit. For 

 this purpose holes were drilled in the tube opposite to each other, 

 at a distance of 2 feet 6 inches within the principal focus. Before 

 these holes short tubes were fixed with screws ; in these tubes slide 

 suitable holders for carrying electrodes or vacuum-tubes. The spark 

 is thus brought at once nearly into the axis of the telescope. The 

 final adjustment is made in the following manner : — A bright star is 

 brought into the centre of the field of an ordinary eyepiece ; the 

 eyepiece is then pushed within the focus, when the wires or vacuum- 

 tube can be seen across the circle of light formed by the star out of 

 focus. The place of discharge between the electrodes, or the middle 

 of the capillary part of the vacuum tube, is then brought into the 

 centre of the circle of light. The vacuum-tubes are covered with 

 black paper, with the exception of a space about a | inch long in the 

 middle of the capillary part ; through this small uncovered space the 

 light passes to reach the slit. 



The accuracy of both methods of comparison, that by reflection 

 and that by the spark within the tube, was tested by the compa- 

 rison of the three bright lines of magnesium and the double line of 

 sodium with the Fraunhofer lines b and D in the spectrum of the 

 moon. I greatly prefer the latter method, because it is free from 

 several delicate adjustments which are necessary when the light is 

 reflected and which are liable to be accidentally displaced. 



Spectroscope A is furnished with a single prism of dense glass 

 with a refracting angle of 59° 42', giving 5° 6' from A to H. 



Spectroscope B has two compound prisms of Mr. Grubb's con- 

 struction, which move automatically to positions of minimum devi- 

 ation for the different parts of the spectrum. Each prism gives 

 about 9° 6' for minimum deviation from A to H. 



Spectroscope C is furnished with four similar prisms. 



The small telescopes of the three spectroscopes are of the same 

 size : diameter of object-glass \\ inch ; each is furnished with three 

 eyepieces magnifying 5*5, 9"2, and 16*0 diameters. 



Spectrum of the Nebula of Orion. 



With spectroscopes A and B four* lines are seen ; they are repre- 

 sented in the diagram which accompanies this note. The scale in 

 the diagram gives wave-lengths. 

 * The fourth line was first seen in nebula 18 H. IV. (Phil. Trans, 1864, p. 441). 



