220 HOW TO WORK 



so great as to spread them over too wide a space and make them very 

 obscure, as is the case when the dispersion is great. Since the light 

 which passes- through, the opening at (/) is not spread: out over the 

 same surface as that which passes through-, the object-glass, it would 

 be far too bright, unless modified by means of a small shutter, 

 opening and shutting with a screw. In each case this can be easily 

 adjusted, so that the light from the two sources is equal, or may be 

 made to vary for some special purpose. There is also a contrivance 

 shown in fig. 320, which enables us to limit the length of the 

 slit ; so that when very small objects are examined, no light shall 

 pass except that which has come through them. 



In using the spectroscope, a great deal depends on the slit being 

 made up of a proper width. If the light be strong., it is best to have 

 the slit only opened so much as to give a. good, clear spectrum, free 

 from the irregular shading, due to unavoidable- irregularities in the 

 slit itself, which may be veiy conspicuous if the slit be very narrow. 

 If day light be employed, and it is only rather feeble, the slit should 

 be made wider, so as to admit more light ; but then, if made too wide, 

 the colours of the spectrum lap over one another, and become indefi- 

 nite. Much, however, should depend on the nature of the object 

 under examination ; and, if it gives rise to very narrow absorption 

 bands, the slit should be made narrow in order to give good defini- 

 tion. As a general rule the slit should be of such a width as to just 

 indistinctly show the- Frauenhofer lines in day light. It is also im- 

 portant to properly adjust the small slit under the side stage attached 

 to the eye-piece. It should generally be made of such a width that 

 the two spectra are of equal brilliancy, since otherwise the compari- 

 son would be inaccurate. 



It is in all cases most important that no. light should pass up the 

 microscope, that has not actually passed through the substance under 

 examination. If the object is small, unmodified light passes on each 

 side, and this is reflected from the front of the object-glass down on 

 the object and back again through the lenses without traversing its 

 substance ; and thus an. entirely false spectrum may be obtained, 

 especially if the substance is dark coloured. This can easily be 

 avoided by having a tube to fit over the object-glass, see fig. 324, 

 which has a stop at the end with a hole in the centre (,tf), of such a 

 width as not to limit the field of the microscope, placed at such a dis- 

 tance as to be within the focal length, so as to approach but not to touch 

 the object when it is in focus. Fora i^-inch object-glass the opening 

 should be about T V inch. Such a stop is also very useful in ordinary 

 microscopical observations, when it is desirable to have no reflected 

 light, and shows incomparably better the true colour of dark objects. 



