102 MICROSCOPES FOR 



object-glass, and placing the miiTor on one side so as to obtain obliqtte 

 light, I have seen, on a clear day, and without any trouble or any 

 complex apparatus, most of the diflBcult lines and dots on the shells 

 of the Navicula, such as those represented in Plates XIX and XX. 



I am fully aware that a microscope may be too small for real work, 

 but the opposite extreme, too often exists now, and where an obserrer 

 can have recourse to two microscopes, he will always find it pre- 

 ferable to making one do all the work. 



Whether the examination of Infusoria be for the purpose of amuse- 

 ment or investigation, the proper illumination of them is of much 

 importance ; in the first place, improper illumination not only pre- 

 vents an agreeable view to be obtained, but greatly fatigues and 

 injures the eye ; while, for the investigation of the true structure of 

 an organism, the proper illumination is of vast moment. The different 

 methods of illimiinations may be enumerated as foRows : 1st. Direct 

 light — either simple, or condensed and unrestricted, or restricted by 

 diaphragms or stops. 2nd. Ohliqtie illumination — either simple or 

 condensed. 3rd. Hie hlach ground iVumination. 



Black groimd illumination. The original account of this plan wiU 

 be foimd in the Micrographia published in 1834. Since then it has 

 been variously adapted. The simplest plan is to place the mirror out 

 of the axis, and to direct the Hght from the min-or so as to illumi- 

 nate the field of view, and by gently turning it (while looking 

 through the microscope) until the object only is bright. In this 

 simple way many animalcules are seen to advantage. I have before 

 me some specimens of the Volvox glolator, prepared in 1841, which 

 are sho^vn in this way to great advantage. 



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 accoimt of the method 

 of examining objects in this way, see the Kev. Mr. Eeade's paper in 

 the Micrographia. 



Some obsen-ers disapprove of the use of candle or lamp light, on 

 accoimt of its colour, and consider clear day Hght can alone show an 

 object properly. As, however, artificial hght is sometimes imavoid' 

 able, it has been proposed to obtain white light, by passing the yellow 

 rays from a candle through blue glass, or such media as "will 



