TEST OBJECTS. 483 



be examined by two or more observers, especially such as the 

 Navicula hippocampus and angulata, in which an appearance 

 of lines is given by dots or projections, arranged in parallel 

 rows, or in rows alternating with each other; nothing can 

 more plainly illustrate the importance of this proceeding than 

 figs. 2, 4, 5, in plate ix. ; a number of persons have carefully 

 examined these under the power by which they were drawn, 

 and have set down on paper what they believed the markings 

 to be produced by; some have declared them to be lines, 

 whilst others, whose eyes were more practised, could resolve 

 each line into a series of dots or elevations, as shown in figs. 

 3 and 7. The representations of the several test objects 

 given in plates vi., vii., viii. and ix., will form an excellent 

 guide to the amateur, as to the amount of definition that a 

 good'object-glass of equal magnifying power to that employed 

 in any given drawing should exhibit, as every specimen has 

 been carefully sketched by the camera lucida, and all the 

 markings put in as they were best seen, with glasses of great 

 or small angles of aperture; the former exhibiting them as 

 dots, the latter as lines. 



CHAPTER XXI. 



MISCELLANEOUS HINTS ON THE MANAGEMENT OF THE 

 MICROSCOPE AND MICROSCOPIC PREPARATIONS. 



Apartment. — In the choice of a room for microscopic observa- 

 tion, one on the ground-floor should be selected in which 

 there is a window having a northern aspect, and not over- 

 shadowed by trees or buildings; a firm table is required for 

 placing the microscope on, and in order that the latter may 

 be at all times ready for use, it should be covered over either 

 with a glass or other shade when not employed; many valu- 

 31* 



