1865.] Mr. Gassiot Description of a Rigid Spectroscope. 323 



servatory, although that instrument has nine prisms. The prisms of that 

 spectroscope are, however, hut little more than half the size of those in 

 the rigid instrument now described. It is to the large size of the 

 prisms, and the greater aperture and focal length of the object-glass, that 

 the superior performance of this instrument must be attributed. 



The temperature of the air and of the prisms is thus observed : A 

 third prism (T P, fig. 3), exactly similar to P and P', is mounted on the slate 

 block. This prism has a hole about | an inch diameter and 1| inch deep 

 drilled in it vertically. A thermometer (Plate VI. T, fig. 3) with a fine 

 cylindrical bulb is inserted in this hole, the intervening space being packed 

 tight with copper filings. The upper part is covered with a layer of fused 

 shell-lac. That the half prism may be about the same temperature as the 

 whole prisms, another half is cemented to it. To avoid confusion, this half 

 is not represented in the diagrams. The thermometer, after leaving the 

 prism, is bent at right angles, and is carried across the top of the prisms on 

 a light metal frame. Another thermometer (T', fig. 3), whose bulb is in the 

 air, its object being to denote the temperature of the air around the prisms, 

 runs parallel with that just described. Both the prisms and thermometers 

 are enclosed under a metal cover, for the purpose of equalizing the tempe- 

 rature and protecting them from injury. This cover has a long slip of 

 stout plate-glass let into the upper part of it, through which the thermo- 

 meters can be seen, and their readings observed. The micrometer eye- 

 piece and the cell containing the object-glass are each mounted on distinct 

 iron blocks. The body of the telescope which fills up the intervening space 

 is mounted on two separate iron blocks not connected with those just 

 mentioned. The tube which forms the body overlaps at one end the tube 

 of the eyepiece, and at the other the mounting of the object-glass, but 

 without being in contact with either. By this means the change of length 

 in the body-tube, produced by change of temperature, by far the most con- 

 siderable change we have to contend with, is prevented from exerting any 

 influence on the indications given by the instrument. 



The homogeneous light of the sodium-flame is employed, and the mi- 

 crometer wires are lighted up by the contrivance shown in fig. 4. A portion 

 of the cap on which the knife-edges are fitted is cut away, and the light 

 which is thus admitted enables the wires, and the rack that serves to 

 register whole turns of the micrometer screw, to be seen distinctly *. A 

 whole turn of the micrometer screw values J-p- of an inch, so that the first 

 reading in Table I. might have been written 0'0552. In making remarks 

 on differences in the readings, they will be expressed in this manner, the 

 reading of the line D decreasing with the rise in the temperature of the 

 prisms. 



The object in taking the readings of the D-line, which are appended, was 



* In fig. 4 the bright lines of sodium, as seen in the rigid spectroscope, are repre- 

 sented ; the spaces A, A are cut away, allowing the light from the sodium-flame to enter 

 and illuminate the field enough to render the cobwebs visible. 



