266 THE USE OF THE MICROSCOPE 



smears and sections, using no cov^er-glass. Do not omit to 

 increase the tube length about 30 miUimeters, or use a 

 collar of that length, so as to make up for the absence of the 

 vertical illuminator. The 74 monobromide of naphthalin 

 immersion objective of 1.6 aperture, can be used in this 

 way on thin stained preparations, without covers, such as 

 smears of bacteria, or chromosomes squeezed from their 

 cells. It gives excellent images of bacteria and chromo- 

 meres, with an oil-immersion condenser of 1.4 aperture. 

 A special eyepiece magnifying 15 times has to be used. 

 To obtain more magnification than the resulting 1,110, 

 screw the amplifier from the Zeiss Bitumi eyepiece attach- 

 ment into the end of the drawtube of a monocular, and 

 increase the tubelength to about 196 millimeters, there 

 being also a 30-millimeter collar. 



44. Advantages of Yellow-green Light. — Use an achro- 

 matic oil-immersion objective on Surirella gemma mounted 

 on the cover in hyrax, the cover-glass being 0.16 millimeter 

 thick. Also use a delicate well-stained preparation, such 

 as spirochsetes of the mouth stained with gentian-violet, 

 or pachytene chromomeres of Aloe stained with brazilin. 

 Get the best vision possible with a neutral screen. Then 

 replace this by an appropriate yellow-green screen. Note 

 the increase in definition and resolution. (Spitta, Nelson, 

 Coles.) 



45. Use of Bluish-green Light. — This is given by 

 Wratten screen No. 64 (Minus Red 3), or by Minus Red 

 4, etc. It produces a sharper image with brazilin-stained 

 objects (or those stained with iron-acetocarmine) ; but 

 cuts off much light, and is not comfortable for long-contin- 

 ued visual observation. It lets through mainly green, with 

 blue, and a trace of yellow. A combination of Wratten 

 screens Nos. 56 and 64 gives an excellent green; which, 

 if intense enough, furnishes a light without red and with but 

 little blue or yellow. This gives especially sharp pictures 

 with the iron-brazilin stain on the monocular microscope. 

 (Nelson, Coles.) 



