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Dr. W. Huggins on the 



[June 13, 



plate polished on both surfaces for the pieces of silvered glass. The oppo- 

 site side of the silver plate to that from which the terrestrial light is re- 

 flected to the slit reflects the images formed by the object-glass to the side 

 of the tube where a suitable eyepiece is fixed. This arrangement forms a 

 very convenient finder, for it is easy to cause the image of the star to dis- 

 appear in the hole in the silver plate. When this is the case, the line of 

 light formed by the star falls on the slit, and its spectrum is visible in the 

 spectroscope. This collimator 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 reflection, and to 

 place the source of the light for comparison directly before the slit. For- 

 merly 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 ob- 

 jection 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 fTxed 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 ad- 

 justment 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 

 I 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 comparison 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 construction, 

 which move automatically to positions of minimum deviation for the dif- 

 ferent parts of the spectrum. Each prism gives about 9° 6' for minimum 

 deviation from A to H. 



