ILLUMINATION. xxix 



be strictly observed ; for all extraneous light which would interfere with the distinctness 

 of the image is thus excluded, and the shadow of the pencil and hand does not interfere 

 with or obscure the sketch in progress, which would be the case if the observer's right 

 hand were towards the window. But in daylight the light entering the eye from the 

 window, even in the position above mentioned, will interfere with the observation, unless 

 a preventive be employed, which is to place a screen, either supported upon a stand or 

 fixed to the upper part of the body of the microscope, between the eye and the eyepiece 

 of the microscope and the light. This screen may be made of card-board or thin wood, 

 covered with black velvet. If it be fixed to a moveable arm, like the lens of the side- 

 condenser, it may be easily placed in any convenient position. If to be fitted on the 

 microscope, it may be constructed thus : a piece of stout card-board, of about the size 

 and shape of one of the plates of this work, should have the corners rounded off, and 

 should be bent at a right angle at about the lower one-fourth ; a hole being cut in the 

 middle of the smaller portion, of a size just to fit the top of the body of the micro- 

 scope, a short tube of card-board is then made by sewing or pasting; and this, being 

 fastened in the same way to the circular aperture, serves to keep the screen in posi- 

 tion. The whole is then covered with black velvet. When used, the long flap should 

 be placed towards the left side ; it then shelters the eye and upper part of the eyepiece 

 from the light. A screen of this kind should always be kept upon the microscope ; for it 

 is of the greatest service. A tube made of a roll of card-board, fastened to the inside of 

 the angle of the screen described above, will serve to fix it to the stem of the side-con- 

 denser ; it may then be made to slide upon this axis or stem at pleasure. It is hardly 

 possible to use the high powers of the microscope by daylight without a screen of this 

 kind. 



But few persons have the opportunity of using daylight for microscopic researches, 

 and with the highest powers ordinary daylight is by no means sufficient ; hence artificial 

 light of some kind is called into requisition ; and the most common source of this is an 

 Argand-lamp (Silber's) with oil ; used with or without a side-condenser. For ordinary 

 purposes this answers well, although the best for examining Diatomacese &c. is a paraffine- 

 oil or camphine lamp, especially when stops and very high powers and eyepieces are used, 

 whereby a large amount of light is intercepted. A cheap common beuzoline lamp with a 

 round or, better, a flat wick, is very advantageous, even with the highest powers (^V^h 

 Beale) ; the direct light being used. In Fiddian's lamp, the flame is enclosed in a metallic 

 case, so resembling a bull's-eye lantern, the light escaping from a round orifice only ; 

 hence no extraneous light can reach the eye. The lamp must slide up and down the stem, 

 so that it can be placed at any height ; and it should be furnished with a shade, also move- 

 able. A white-cloud earthenware or enamel shade is often used. Norman's paraffine- 

 lamp, and Collin's Bockett lamp, with an attached bull's-eye, and How's lamp, are good 

 lamps. An improved lamp, for illuniination and centering with high powers, is described 

 by Dallinger (M. Mi. Jn. 1876, xv. p. 165). 



Much of the success with which the structure of an object is displayed will depend upon 

 the manner in which the light is thrown upon or transmitted through it. In general the 

 more light that can be condensed upon opaque objects the better; and when the various 

 parts of such objects are of different colours, the more direct the light and the greater the 

 angular aperture of the object-glass, the more clearly will the parts be distinguishable ; 

 while in certain opaque objects which present questionable elevations or depressions on 



c2 



