72 The Precipitins: Methods 



cloudings or feeble reactions observed, for instance in the generalized 

 mammalian reaction already noted, have certainly been overlooked by 

 other observers because of the mode of illumination employed. For the 

 same reason it is of advantage to have the stands and racks painted 

 black. By moving the riband up and down, the obliquity of the rays 

 of light falling across it from the window and illuminating the test- 

 tubes can be regulated to suit each row of racks, and render slight 

 reactions quite evident. I can but recommend the use of this simple 

 apparatus to those engaged in similar work. 



SOURCES OF ERROR IN PRECIPITIN TESTS. 



1. Opalescent Antisera. 



In the absence of more obvious sources of error, such as I shall 

 mention below, the use of what may be described as " opalescent 

 antisera" constitutes one of the gravest in using the test. A little 

 familiarity in the process of testing, and the making of one or two 

 control tests on heterologous sera, will however suffice to exclude the 

 error which might otherwise be made by using the antisera about to be 

 described. Uhlenhuth (11 18, ix. 1902, p. 680), Miessner, and Rostoski 

 (1902, b. p. 29) have observed these. It was due to the fact that my 

 anti-bear serum (referred to in an earlier paper, 5, IV. 1902) belonged 

 to this category, that I never published any tests made with it. 

 I found that it clouded every blood dilution, mammalian and otherwise, 

 to which I added it. Although it gave a great reaction with its 

 homologous serum, it gave such marked reactions with all other sera, 

 that I immediately discarded it, without being able to explain why 

 it behaved so peculiarly. Since then I have fortunately rarely 

 encountered such antisera. Uhlenhuth has been the first to sound 

 a warning against their use, medico-legally. As he says, a slight 

 opalescence is usually perceptible when any serum or antiserum is 

 added to blood dilutions, the tube being viewed by strong transmitted 

 light, but the clouding here referred to is totally different and much 

 more marked, and takes place even in salt solution. I tried to clear my 

 anti-bear serum by filtration through porcelain, but was unsuccessful. 

 Uhlenhuth has had the same result. We are still in the dark as to 

 the cause of this peculiar milkiness or opalescence of certain antisera. 

 Uhlenhuth thought it might depend upon the stage of digestion at 

 which the animal was killed, but this seems to me unlikely for the reason 

 that I have killed my animals at no fixed time with regard to meals, 

 and on the whole have encountered the condition but rarely. Moreover 



