ORDINARY CONDENSER IMPROVED, &C. 315 



tubes and eyepieces, and on the upper half of the stage opening, so 

 that the light may illuminate both tubes equally. 



In its fixings it will require only the ordinary up and down rack- 

 work, and a means of centreing, or a movement of about half-an- 

 inch or less, so as to carry the slit backwards and forwards vertically 

 when the instrument is inclined for use. This movement, should 

 there be no rectangular sub-stage, may be readily effected from a 

 centre, or turning on a pivot, the distance to be traversed being so 

 small that a radial direction will scarcely at all interfere with its 

 performance. 



In using, the lamp is best placed in front, a little to the right. 

 After placing an object on the stage and getting an approximate 

 focus, the prism should be drawn out and the light well centred to 

 the field ; then replace the prism and, if rightly set, both fields will 

 be equally bright. Next, search for the best part of the flame and 

 adjust the condenser to its position, moving it outwards or inwards 

 until the best effects are obtained. A very little practice, with 

 careful attention, will soon render its use familiar and certain. It 

 will, of course, be important to see that the mirror is perfectly 

 central with the axis, and at such a distance as may be suitable to 

 the intensity desired. 



In the present instance, the lens is about one inch in diameter ; 

 but it has also been tried, with equally good results, with lenses 

 about one-third the size — the proportions of the slit remaining the 

 same. 



In using a lens, in conjunction with a polarising prism, this 

 opening will not afford sufficient light for the purpose, and it has 

 been found desirable to extend the aperture to something more than 

 the half-circle, as in Fig. 4. An angle of about 145° has been ob- 



Fig. 4. 



tained (by using a collapsing and expanding contrivance) as the 

 most efficient angle, and giving an equal amount of light from the 



