POSITION OF THE LIGHT. 157 



neutral-tint glass ; and it will then be nearly purified as to 

 quality, though much reduced in intensity. 1 Mr. Rainey, who 

 has paid great attention to the hest means of obtaining a good 

 illumination by artificial light, recommends, as the best moderator, 

 one piece of dark blue glass, free from any tint of red, another 

 of very pale blue with a slight shade of green, and two of thick 

 white plate-glass, all cemented together with Canada balsam ; 

 this, as already stated, may be used with sun-light, as well as 

 with lamp-light. The Microscopist who wishes to render the 

 artificial light which he may be in the habit of using, as pure as 

 possible, will do well to compare with daylight (as suggested by 

 Dr. Griffith, who seems to have been the first to employ tinted 

 glass with this object) ; furnishing himself with several pieces of 

 glass of different shades, substituting one for another, and alter- 

 ing their distances from the lamp, 2 until he has succeeded in so 

 tempering its rays, that the field of his Microscope, or the object 

 under view, is not more colored when illuminated by the artificial 

 light reflected from the mirror, than it is when the mirror is so 

 turned as to reflect good light from a white cloud. 



76. Position of the Light. When the Microscope is used by 

 daylight it will usually be found most convenient to place it in 

 such a manner, that the light shall be at the left hand of the ob- 

 server. It is most important that no light should enter his eye, 

 save that which comes to it through the Microscope ; and the 

 access of direct light can scarcely be avoided, when he sits with 

 his face to the light. Of the two sides, it is more convenient to 

 have the light on the left; first, because it is not interfered with 

 by the right hand, when this is employed in giving the requisite 

 direction to the mirror, or in adjusting the illuminating appara- 

 tus ; and secondly, because, as most persons employ the right 

 eye rather than the left, the projection of the nose serves to cut 

 off those lateral rays, which, when the light comes from the right 

 side, glance between the eye and the eye-piece. In order to pre- 

 vent, still more completely, the access of false light, it is desirable, 

 if it be otherwise convenient, that when daylight is employed, 

 its source should be a little behind the observer : but as it will 

 then, by falling upon the stage, interfere with the view of any 

 object which is imperfectly transparent ( 87), it maybe necessary 

 to keep it from doing so, by the interposition of a screen. "When 

 Artificial light is employed, the same general precautions should 

 be taken. The lamp should always be placed on the left side, 



1 A gas-lamp provided with these and other appurtenances for regulating the illumi- 

 nation, and also with a water-bath and mounting-plate, is supplied by Mr. S. Highley, 

 Fleet Street. 



2 The nearer the colored glass is approximated to the flame, the less modification will 

 it produce in its rays ; since their intensity varies in different parts of their course, in- 

 versely with the square of their distance from the illuminating centre, whilst its in- 

 fluence is a constant quality. Hence a pale-blue glass placed near the mirror, or be- 

 tween the mirror and the stage, has more effect than a chimney of much deeper blue 

 immediately surrounding the flame. 



