68 



The Precipitins: Methods 



The test-tube racks (Fig. 4) were made in the laboratory, and consisted 

 of a flat strip of wood upon which three equidistant blocks of wood were 

 nailed to support a strip of cardboard perforated with holes of suitable 

 size 2 cm. apart. The holes were bored by means of a red-hot iron rod 

 pointed at one end. Each rack contained holes for 20 tubes, the strip 

 below being indented by means of a nail-punch with holes correspond- 

 ing to those above in the cardboard ; these indentations received the 

 convex bottoms of the small tubes and fixed them in place. Each rack 

 was lettered according to the alphabet at one end, and numbers 1 to 20 

 were written in ink upon a strip of white celluloid which was tacked 



, m 



i**~- _ 



nnnn 







- * 



-1.-K.W T. f " - - -. f "^ fi^rf- 



-g-T~, < n t >?. -. T .-^ T . ., . .. .y ^t 



Fig. 4. 



Showing the test-tube racks on the stand, and a single rack in the foreground. The 

 black riband is shown at the top, forming a background for the top row of tubes. For a 

 further description see the text, p. 70. Every part of the stand and racks, except the 

 celluloid strip bearing the numbers, is blackened. 



(The figure is reduced to f$ natural size.) 



