APPENDIX. 



385 



There are several important points of detail that must be attended to 

 before accurate results can be obtained. 



(1) See that the zero points of the scales -... 

 are correct, by carefully lowering the plungers o *& 

 until they touch the cups and noting the 



readings. 



(2) See that the prisms and eye piece are 

 clean. Specks of dust seen in the field are apt 

 to lead to erroneous judgments. The prisms 

 can be removed and carefully cleaned with a 

 pointed match covered with two or three layers 

 of silk. Great care must be taken to avoid 

 breaking the prisms away from the cement. 



(3) See that the illumination of the two 

 fields is equal. This is b,est tested by placing 

 a coloured solution (such as a 2-5 per cent, 

 solution of potassium dichromate) in both cups, 

 placing one plunger at 15 mm. and adjusting the 

 other until the two fields have an identical 

 appearance. The other plunger should also 

 be at 15 mm. Attention must be paid to the 

 point considered below. 



(4) Folin has suggested that the best 

 place for an instrument is in the middle of 

 the Laboratory, so that the eye is not dazzled 

 by the light from the window. Retinal fatigue 

 will undoubtedly cause very serious errors and 

 inconsistencies, and Folin's recommendation is 

 valuable. 



(5) A comfortable body position is im- 

 portant. For some reason the best results are 

 obtained when the observer is in an unstrained 

 position. Folin suggests that the best way of 

 using the apparatus is to place it on a stool about 

 the same height as an ordinary chair, and to sit 

 at the side of the instrument. His method of 

 reading the unknown is to place the standard 

 solution into both cups and to set them at the 

 same height. The instrument being adjusted, 

 both fields should look alike, and the eye gets 

 accustomed to the appearance. The standard 

 in one cup is replaced by the unknown, and 

 one very careful observation is taken. When 

 making a series of comparisons, he re-reads the 

 standard against itself after each of two un- 

 knowns. 



Kober's Colorimeter* is a great advance on Duboscq's. The manu- 

 facturers (Klett Manufacturing Co., New York, U.S.A.) kindly sent one to 

 Cambridge for trial. It has been found admirable in every way, and is always 

 used now in preference to the various patterns of Duboscq's. Kober's 

 instrument can also be used as a Nephelometer, i.e. for estimating substances 



Fig. 53. Diagram of path 

 of rays in Duboscq's 

 Colorimeter. Below 

 are representations of 

 the appearance of the 

 field under different 

 conditions, that on the 

 left with no fluid in B, 

 and that on the right 

 when the tints are 

 matched. 



* Journ. of Biol. Chem., xxix., p. 155. 



BB 



