EXPERIMENT STATION BULLETINS. 



561 



and forth until a tube is found that compares in color with the first unknowm 

 in the second rack A. The rack containing standards is moved back and 

 forth by pulling the strings sho^vn in the cut. The second rack A carrying 

 the unknowns is then moved along one place and the reading of the second 

 unknown found in the same way. The under surface of each sliding rack 

 is indented opposite each test tube, the indentation being designed to re- 

 ceive a cog supplied with a spring. By means of this arrangement the tubes 

 arc stopped with a click at the proper place in the path of light as the racks 

 are moved back and forth. Rack B is so constructed that it can be removed 

 readily and replaced by other racks containing standard tubes for other 

 ranges of pH. 



The readings are made by using light directly transmitted or the compa- 

 rator is tilted by placing books under the end toward the observer and light 

 reflected from a white surface through the tubes. 



It is possible by using this comparator to examine, in a given time many 

 times the number of samples which could be examined by the old method. 

 This comparator is now being placed upon the market by the Central Scien- 

 tific Company of Chicago. 



The comparator described above is indispensable to the worker wishing 

 to examine a number of samples of milk at a time. For one who examines 

 only a few samples, the combined comparator and storage rack, designed 

 by Cooledge and Klever will be sufficient. (Fig. 3). 



Two strips of wood 13^ inches by 3 inches by 14 inches are selected and 

 thirteen ^ inch holes sunk in one edge of each. One of these is used as the 



UGHT 



EYE 



Fig. No. 2. Detail of comparator shown in Figure No. 1. 



