194 USE OF THE MICROSCOPE. 



position, having the stage h at right angles to it ; the mirro r 

 m is turned on one side, so that the diverging rays r r' 

 emanating from the lamp /, may pass through the hole in the 

 stage in nearly straight lines ; the same position of the instru- 

 ment will sometimes answer for daylight, but it will be gene- 

 rally found that, to get the brightest Hght, the compound 

 body must be more depressed. If the lamp employed as the 

 Uliiminator can be slid down so far, that the end of the burner 

 nearly touches the table, or if a short piece of wax candle be 

 used, the compound body may be inclined at an angle the 

 most easy for the observer to work at ; the incUnation in all 

 cases must, of course, depend upon the height of the illumin- 

 ating body from the table on which the microscope is placed. 



Oblique Light. — For the perfect definition of the markings 

 of certain animalcules of the genus Navicula, very oblique 

 light must be employed, which may be effected by placing the 

 microscope in the inclined position, and taking away all the 

 apparatus from the under surface of the stage, such as the 

 diaphragm-plate, or achromatic condenser, and by turning the 

 mirror so far on one side, that the rays of light may be thrown, 

 as it were, obliquely across the object, and not perpendicularly 

 through it. When the mirror cannot be turned very far away 

 from the centre of the stage, the desired efiect may be ob- 

 tained by mounting the object upon something that will raise 

 it some distance above the general level of the stage. Mr. 

 Anthony employed two of the boxes which contain the object- 

 glasses for this purpose. 



Background Illumination. — The attention of microscopists 

 was first drawn to a method of illumination by the Rev. J. 

 B. Reade, in the year 1838, who called it by this name. The 

 method consists in illuminating the object by a very powerful 

 light, placed at such an angle with the axis of the microscope 

 that none of the rays can enter it, except those which fall 

 directly upon the object, and are so far bent as to pass through 

 it into the compound body. For the better understanding 

 of the mode of effecting this illumination, fig. 123 has been 

 drawn to give a general idea how the microscope and the 

 other apparatus are to be placed for the purpose : a h re- 



