44 



THE MICROSCOPE 



Relative 

 intensities 

 of different 

 sources of 

 illamination. 



the ordinary type is unsatisfactory unless used with a ground 

 glass or tissue paper in front of it. A form of 1/2- watt bulb 

 called the " Grid " is a good light, as the filaments, when looked 

 at from the correct side, appear as a fairly large ribbon of almost 

 homogeneous light. The " Pointolite " electric arc is extremely 

 good for the highest power work. The incandescent gas mantle 

 lamp is a useful illuminant, and a modification of this, heated 

 by a methylated spirit lamp, is an excellent light for those who 

 have not gas or electricity. 



The relative intensities of a similar size small area of different 

 illuminants are approximately according to the following table 

 taken from Mr. A. P. Trotter'^ book on Illuminating Engineering : 



Candle 



Daylight (blue sky) 



Paraffin lamp 



Incandescent gas mantle 



Carbon electric filament 



Metal electric filament 



1/2-watt electric biilb . 



" Pointolite " electric bulb 



Arc lamp , 



Direct light from the sun 



2i 

 2 



4 to 9 



50 



300 



1,000 



5,250 



12,000 



80,000 to 110,000 



. 800,000 



From the above table it is evident why ordinary daylight is 



not sufficiently powerful for high- power 

 microscope work. The sun, even in a clear 

 climate, requires the use of a heliostat, and 

 the arc lamp requires a special equipment. 

 It is of great advantage to use a very in- 

 tense light modified with the neutral tint 

 wedge moderator described on page 32. 

 The brightness of the light can then be 

 perfectly regulated to meet requirements. 



This advantage of a very powerful illu- 

 minant has been referred to in connection 

 with substage condensers and dark-ground 

 illuminators, but care should be exercised 

 in its use. When direct light 

 is being used through a con- 

 denser, it is damaging to the 

 eyes if too strong a light is em- 

 ployed. A strong illuminant is 

 necessary for high- power dark- 

 ground or opaque illumination, 

 but it must be modified when 

 direct light is thrown through 

 the object. Some colour screens 

 require a strong light, but imme- 

 diately they are removed the 

 Just enough light to show the 



Fig. 36. 



-No. 3335, Paraffin 

 Lamp. 



hght should be cut down. 



