4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



closing the window, thereby isolating the working room from the 

 source of ultra-violet light. The standard plate holder mounting is 

 shown at the left while the special biological plate holder lies at the 

 front of the concrete table. It is shown with the back removed. Slide 

 and quartz window are placed at 45° to the plate support in order to 

 minimize reflection from the window. 



l^ 



'^'. 



I I I I I I I I I 



.3 5 .40 



.25 .30 .35 .40 .50 1.00 r.SO p 



Fig. 2. — Record of mercury spectrum taken with open thermocouple. 



Plate 2 shows the camera, thermocouple, and recording system in 

 the adjoining room. The biological plate holder is in place with the 

 back and slide removed. The thermocouple can be moved across the 

 spectrum imder these conditions, the traverse thus being made under 

 the identical conditions to .which the biological material is exposed. 

 At the extreme right the box is shown w^hich houses a cylinder upon 

 which the galvanometer record is made through a cylindrical lens. Gear 

 mechanisms accomplish convenient reduction of motion so as to secure 

 the desired spread of spectrum upon the record. An interchange of 

 standard reduction gears allows a modification of ratio. 



Figure 2 is a direct record of the mercury spectrum from 2,500 A. 

 to i.7fj.. This record was made with an open thermocouple, thus 



