96 



PROTOPLASM 



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The Ultramicroscope. — That colloidal systems consist of 

 minute particles in permanent suspension was early suspected. 



Faraday suggested this when 

 he first showed a cone of light 

 in colloidal gold to the Royal 

 Society. Proof of the presence 

 of such particles is to be had 



Fig. 71.— Lateral , i.e., indirect (dark- in ultramicr0SC0pic,0T dark-field, 

 field) illumination. .,, . . . r^\^ • ■ i c 



Illumination. 1 he principle oi 

 the ultramicroscope is the same as that of the Tyndall cone. 



Fig. 72. — The ultramicroscope. (Carl Zeiss.) 



In both cases, colloidal matter is illuminated laterally against 

 a black background, with the result 

 that microscopically invisible particles 

 become "visible" because of the light 

 that they scatter. The direct beam 

 of light from the source of illumina- 

 tion does not enter the eye of the 

 observer; only the scattered rays 

 from the particles are seen (Fig. 71). 

 The colloidal particles thus illuminated 

 appear as brilliant spots against a 

 black background. Two main types 

 of lens systems are used to give 



lateral illumination — the cumbersome Fig. 73.— The cardioid con- 

 equipment known as the MZ^ramz'croscojse denser. (Arthur H. Thomas.) 

 (Fig. 72) and the simple cardioid condenser, or dark-field illumi- 

 nator (Fig. 73). 



