54 THE MICKOSCOPE. 



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

 thatr 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 fan-shades 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 upon the eye-glass instead of through it. Take care 

 also that the mirror is properly 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 effected 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 



