INFUSORIAL ANIMALCULES. 49 



convinced tliat some very important results would be ob- 

 tained. The ribs or striae on the navicula assume a slight 

 tinge of colouring when the polarizer and analyzer are 

 parallel to each other ; but when they are crossed, owing 

 to the few rays which are transmitted, I was unable to 

 perceive it. Isthmia are slightly influenced by polarized 

 light. The larger Infusoria I have not examined, nor am 

 I aware that any information is recorded on this subject. 



Section XXI. — On Vieiving Infusoria by means of the 

 Black Ground Illumination. 



The muscular fibres of the Rotatoria, and the markings 

 on the lorica of the Bacillaria, &c. are brought out in a 

 most remarkable manner by this mode of illumination. 

 For an account of the method of examining objects in this 

 way, see Microscopic Illustrations, p. 138. 



Section XXII. — Classification of Infusoria. 



Among the various arrangements proposed for the dis- 

 tribution of animalcules by different naturalists — and we 

 have not a few, as the minuteness of these creatures and 

 the imperfections of our microscopes, until lately, allowed 

 ample field for the imagination to run wild — two only 

 appear to me to merit particular notice, and these, it is 

 worthy of remark, are the productions of men who have 

 laboured for years in making actual observations on them. 

 The first is by O. F. Midler, whose posthumous work, entitled 



E 



