CHAPTER VIII 

 MICRO-SPECTROSCOPE AND POLARISCOPE 



§ 390. Apparatus and material for Chapter VIII. 



i. Compound microscope. chlorophyll, some colored fruits, 



2. Micro-spectroscope (§ 392,400). etc. (§ 412-419). 



3. Watch-glasses and shell vials, 5. Micropolarizer (§ 421). 

 slides, and covers (§ 410). 6. Selenite plate (§ 431). 



4. Various substances for exami- 7. Various doubly refracting ob- 

 nation (as blood and ammonium jects, crystals, textile fibers, starch, 

 sulphide, permanganate of potash, section of bone (§ 430). 



§ 391. Visible and invisible radiation. — From any primary- 

 source of light energy like the sun, the electric arc, etc., not only is 

 the energy which to the eye is appreciated as light, but wave lengths of 

 energy both longer and shorter than those affecting the eye are 

 given off. As shown in fig. 144 the segment of the energy spectrum 

 which is visible to the eye is exceedingly limited, being included 

 between about \0.41j. and \0.7n. Under special illumination, waves 

 shorter than \0.41j, and longer than X0.7/X can be seen, but the exten- 

 sion into the infra-red or the ultra-violet is very slight, and not used 

 for ordinary visual purposes. 



It is fortunate for optical instruments that the visible spectrum is 

 so limited. Indeed, if the visible spectrum were even more limited 

 it would be easier to obtain perfect images, for the aberrations arising 

 from the different wave lengths would be so much the less. 



The spectroscope has for its object the giving of information con- 

 cerning the visible spectrum, and it has proved of very great help 

 indeed. It should not be forgotten, however, that the color effects 

 produced by the spectroscope are not the only ones and in some ways 

 not the most important. What it really does is to divide up the 

 wave lengths in groups, and in absorption phenomena the important 

 thing is that some wave lengths are not present or are cut out by the 

 absorbing medium and hence there are present dark bands in the 



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