118 BELL SYSTEM TECLINICAL JOURNAL 



for pholomicrography and has a 5 x 7 camera of rather short bellows 

 extension. The objective equipment is of the achromatic type. 



For transparent work a Zeiss \'ertical camera outfit, Fig. 17, equipped 

 with the con\entional Zeiss research type microscope is used. The 

 camera has a bellows extension of 80 centimeters and uses 5x7 or 

 smaller plates. It is fitted with ground and clear glass focusing 

 screens similar to the large Zeiss metallurgical outfit. The illuminanl 

 is a 500 watt metal filament nitrogen filled bulb with the filament 

 mounted so that a large circular area of illumination is presented, or 

 if desired, the filament assembly may be turned sideways and a single 



rig. 18 — .\ 500 wall metal filament, gas-filled lamp for use in pliotomiorograpliy. 



filament strand is thus presentcil to the ojitical train. In niediinn 

 and high powers, this approximates a jioint source and lor the lower 

 powers the large circular arrangement of ihi.' Iihinunl pro\'ides a 

 relatixely large area of illumination wliirii is quite desirable. Tiie 

 lam[) is illustrated in Fig. 18. The iliiniiinating train consists of 

 condensing and cooling units adjustably moimled on a substantial 

 optical bench as in the case of the metallographic outfit. The ob- 

 jectives consist of a full set of apochromats and also se\eral achromats 

 of low power. The niicro-planars are also used with this equipment. 



The Ultra-Microscope 



The ultra-microscope is an instrument for re\ealing the pre.sence 

 of very minute bodies present as colloids in transparent solids or 

 liquids. The presence of these particles is made ajjparent by the 

 light rays which they intercept and dilTract upward into the micro- 

 scope objective. It is a matter of common obser\ation that dust 

 particles are seen in an intense beam of light such as sunlight but 

 otherwise their jiresencc remains concealed. This principle of il- 

 lumination is made use (jf in the ultra-microscope as described below 

 and accordingl>' dilTers considerably from the con\enti(in.il arrange- 

 ment of compound nncroscope and illuminant. 



The appearance of ultra-microscopic particles in fluids and trans- 

 parent solids as seen by means of the ultra-microscope is, without a 



