138 



MICROSCOPIC ILLUSTRATIONS. 



. SECT. 17. The Black Ground Illumination for Trans- 

 parent Objects. The Rev. Mr. Reade, in the Appendix 

 to the Micrographia, has described a method of illumi- 

 nation for transparent objects, differing from any at 

 present in use. It consists in illuminating the object by 

 a very strong light placed at such an obliquity to the 

 axis of the microscope that none of the rays can enter it, 

 except those which impinge directly upon the object, and 

 are thereby bent so as to pass through the object into 

 the instrument. As the arrangement for viewing objects 

 in this way may not readily suggest itself from a verbal 

 description only, it has been thought advisable to illus- 



Fig. 29. 



