174 PHYSIOLOGY OF THE INVERTEBRATA. 



lower end of B) the parallel rays proceeding from the prism 

 are again brought to a focus, and this real spectrum is 

 observed by an eyepiece L. By two slit mechanisms at 

 right angles to each other, actuated by the screws tt\ uu' in 

 the focal plane of the eyepiece, the visual field can be limited 

 at pleasure according to the procedure of Vierordt. 



By means of two lenses shown at an image of a wave- 

 length scale is projected on the spectrum by reflection from 

 the end-surface of the Amici prism, which is illuminated by 

 the mirror S' and put out of action by closing the shutter d'. 

 Adjustment of this scale is made by inclining the whole 

 scale-tube with the screw w, which is opposed by a counter- 

 spring V (Pig. 33). 



Both, of these instruments are of the utmost importance for 

 investigating the chromatology of the Invertehrata. 



Although Dr. MacMunn * uses the microspectroscope, he 

 says that, " when the amount of material is sufficient for the 

 purpose, it is best to measure the position of bands by the 

 clieviiccd spectroscope and reduce the readings to wave-lengths 

 by means of a curve plotted out on logarithm paper, as 

 directed in Watts' Index of Spectra. Similarly, the readings 

 of others can be reduced to wave-lengths by laying a scale — 

 say of millimetres — along the top of their maps, and noting 

 the readings of the Fraunhofer lines, and then, by means of 

 an index of spectra (such as Watts'), finding the wave-length 

 of these lines, and laying them down in accordance with 

 these data on the logarithm paper. One can also detect an 

 error in the map of any observer by this method. ' So 

 delicate is this graphical method of detecting error, that 'by 

 its means we might very readily detect error in tables 

 of logarithms or trigonometrical functions.' In using the 

 diSraction grating it is nearly a straight line, and Sir George 

 Stokes, P.R.S., says that by using the reciprocals of the 

 wave-lengths instead of the numbers themselves, one has a 

 straight line instead of a curve." . 



* Proceedings of Birmingham Philosophical Society, "vol. 5, p. 180. 



