MICROSCOPIC OBJECTS. 119 



satisfactorily to demonstrate its structure ; but it appears 

 to consist of two delicate tissues having regular rows of 

 conical spines on the upper one. As a test-object these 

 spots should be clearly and distinctly separated." The 

 spots on the " hen-coop" scales, when viewed under a 

 very deep power, appear quadruple ; each spot, including 

 the four, measures y^ Jo o^^ ^^ ^^ ^^^^- " When the hght 

 is thrown obhquely, they are blended together, appearing 

 like a stripe of unequal breadth. Similarly to many of the 

 other tests, it is the manner in which they are seen, 

 rather than the mere exhibition of them, that should be 

 observed. This object I employ for the same purpose as 

 the leaf of an unknown species of moss belonging to 

 the genus Hypnum, which, as it is difficult to procure, 

 renders this substitute an acquisition to the micro- 

 scopist." 



Note. — With respect to illumination," the reader should 

 observe that quantity and intensity are distinct from each 

 other ; thus when we employ a small wax taper close to 

 an object, it will be intensely illuminated though the 

 quantity of hght is small ; but if we employ the flame of 

 a large lamp, &c. at some distance from the object, its 

 intensity will be small though the quantity of light be 

 great. It will be found genergdly preferable to employ a 

 small quantity of intense light rather than aiarger portion 

 of weak hght, and, if possible, avoid the use of lenses or 

 mirrors either for condensing or changing the direction of 

 the hght. This remark applies especially to the verifi- 

 cation of an object. 



M 2 



