254 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



resistance coil is supplied for use with currents of 65 to 220 volfcs- 

 A current strength of about 3*5 amperes is necessary. As seen in 



fig. 36, the tube is placed in an inclined position in a 1 l-shaped 



holder and secured with springs. The lower end of the lamp where the 

 mercury accumulates is connected with the negative pole of the circuit. 

 The lamp is shielded from the observer by an iron shade, so as to shut 

 off the superfluous light ; the shade has an opening opposite the middle 

 of the lamp for transmitting the light, which then impinges upon the 

 flask filled with a solution and acting as an engraver's globe. The holder 

 of this flask (see figure) also acts as a handle for lifting the whole appa- 

 ratus. The effect of the entire arrangement is to throw an image of the 

 illuminating mercury column on the iris diaphragm of the Microscope, 

 and hence on to the plane of the object. The fluid in the flask not only 



Fig. 36. 



provides light for the lenses but also acts as a light filter. For clear 

 green light of wave-length 546 /a/a (this is the brightest line of the mer- 

 cury vapour lamp) the filter should have the following composition :— 

 distilled water, 300 c.cm. ; picric acid, 0'4 grin. ; copper sulphate, 

 3-5 grm. ; didymium nitrate, 15*0 grm. If this didymium nitrate 

 be omitted no light is transmitted beyond the line A = 546 /a/a ; the 

 yellow lines A = 576 /a/a and 579 /a/a also disappear. These two yellow 

 lines are, however, obtained with great brightness with the following 

 solution : — distilled water, 300 c.cm. ; potassium bichromate, 15 grm. ; 

 copper sulphate, 3*5 grm. ; sulphuric acid, 1 c.cm. The blue and violet 

 lines A = 436 /a/a, 407 /a/a and 405 /a/a are obtained with : — distilled water, 

 225 c.cm. ; copper sulphate, 1 grm. ; ammonium hydrate, 75 c.cm. With 

 subjective observation only the line 436 /a/a is effective, as it is much 

 more intensive than the two violet lines ; the light in this case is, there- 

 fore, practically monochromatic. Flasks can be filled with the respective 

 solutions and secured with well paraffined corks ; they are then always 

 available for obtaining light of their corresponding colours. For the 



