BAIRD ^TXTLOCK (LONDON) LTD. 



THE HILGER WAVE-LENGTH SPECTROSCOPE, 1907 MODEL. 



The latest form of this spectroscope is shown in Figs. 5802-5807. The telescope and collimator are of II \ in. 

 focal length and i J in. aperture. The prism is of a special form, and may be best considered as built up of two 

 30 prisms and one right-angled prism from the hypotenuse of which the light is internally reflected, as in Fig. 2. 

 The prism is, however, actually made in one piece. The table on which the prism stands is rotated by means of a 

 fine steel screw, the point of which pushes against a projecting arm on the prism table. To the screw is fixed 

 a helical drum (see Fig. 3} , on which the wave-lengths of the line under observation are read off direct as indicated 

 by the index, which runs in a helical slot. The point of the micrometer screw is of hardened steel, which is finally 

 fixed before the turning of the screw thread is done, to avoid risk of periodic errors ; the cutting of the screw thread 

 being done on this point as one of the centres. This hardened steel point presses against a steel plug in the 

 above-mentioned projecting arm of the prism table and this steel plug is flint hard, and is optically polished. 



The telescope and collimator are both very rigidly fixed to the cast-iron base, and the whole is screwed to a 

 strong cast-iron tripod. 



It will be seen that the design is extremely strong and simple ; and the accuracy is as great as that obtainable 

 by the use of the very highest class of divided circle spectroscope (unless recourse is had to careful line-to-line 

 measuiements with a micrometer eyepiece or some similarly laborious process, while in ease, speed, and convenience 

 the " Wave-length " form is vastly superior. 



Fig. I 



Fig. 4 



Fig. 2 



Fig. 5 



Fig. 3 

 5802-5807 



5802 T Wave-length Spectroscope, prism of 1.65 refractive index for D, accurately calibrated 

 throughout the whole visible spectrum 



5803 T With denser prism, 1.74 refractive index for D, and correspondingly increased accuracy 

 of calibration 



Accessories for Use with the above Spectroscopes. 



5804 T Protective cover to prism table 



5805 T Camera, with 21 in. focus lens, and tilting for accurately focussing the whole spectrum 

 (shown in position in Fig. 4) 



5806 T Reading telescope for reading off the drum (by means of a double reflecting prism the 

 figures are not reversed) on raising and lowering stand 



5807 T Shutter eyepiece with lateral adjustment to bright pointer (see Fig. 5, and description 

 below) 



NOTE. The above eyepiece is of very great assistance in the mapping of spectra. By the aid of two shutters, 

 vhich can be shifted in from either side at will in the focal plane to cover any desired part of the field, any bright 

 Bines which by their proximity to feebler lines obscure the latter can be hidden from view. The metal pointer, 

 Evhose point is ground exceedingly fine and polished bright with the greatest care, is illuminated from above with 

 111 mirror. This bright pointer is adjustable laterally by the two milled head screws below, so that one can always 

 ( eturn to the standard reading by setting the bright pointer on a reference line. 



27 10 



30 10 



13 



776 



400 



4 12 6 



1 



CHEMICAL AND SCIENTIFIC APPARATUS AND PURE CHEMICALS. 



1053 



