5.4 THE MICROSCOPE. 



when the object is not on the slider-holder : if you do not, it is a sign 

 that something is wrong ; perhaps the body is not placed directly before 

 the aperture of the slider-holder, or may not be truly directed towards 

 the light. Use the least amount of light possible, if you work for any 

 length of time. Choose a steady light, with a shade to protect the eyes; 

 one of the old-fashioned /aw-sAoc^es will be found useful for this purpose. 

 Look at the object with both eyes open, and use the eyes alternately. 

 Sit in a comfortable position, and bring the instrument to the proper 

 angle, which will prevent congestion of the eyes; this is indicated 

 if the microscopist is annoyed with little moving objects apparently 

 floating before them : if the eye-lashes be reflected from the eye-glass, 

 you are looking v^pon the eye-glass instead of through it. Take care 

 also that the mirror is propra-ly arranged. 



Sir David Brewster's excellent directions for viewing objects should 

 be made familiar to the microscopist. He observes : 



" First. Protect the eye from all surrounding light, letting only the 

 rays which proceed from the illuminated centre of the object fall upon it. 



" Secondly. Delicate observations should not be made when the 

 fluid which lubricates the cornea is in a viscid state, or there is any 

 irritation or inflammation about any part of the eye. 



" Thirdly. The best position for microscopic observations is with 

 the microscope bent to such an angle with the body, that the head may 

 always remain in a natural and easy attitude ; consequently, the worst 

 position would be that which compels us to look downwards vertically. 



" Fourthly. If we lie horizontally on the back, parallel markings 

 and lines on objects will be seen more perfectly when their direction is 

 vertical, or in a contrary direction to that in which the lubricating fluid 

 descends over the cornea of the eye. 



" Fifthly. Only a portion of the object should be viewed at one time, 

 and every other part excluded. The light which illuminates that part 

 should be admitted through a small diaphragm : at night, from the con- 

 centrated light of a sperm-oil or gas lamp, having a faint blue-tinted 

 chimney-glass to correct the yellow colour which predominates in all 

 our artificial illumination. If in the day-time, close a portion of the 

 window-shutters. 



" Sixthly. In all cases when high powers are used, the intensity of 

 the illumination should be increased by optical contrivances below the 

 object and stage : this is generally efiected by using achromatic con- 

 densers beneath the stage." 



Mr. James Smith contributed the subjoined practical observations 

 on the same subject to the Microscopical Transactions : Much of the 



