136 ARRANGEMENT OF APPARATUS. 



The last method requiring description is that in 

 which sunlight is used. 



Any of the previous arrangements may be em- 

 ployed if the apparatus is situated so as to allow the 

 light from the heliostat to fall on the condenser. 



It will be found advisable when the distance be- 

 tween the microscope and reflecting mirror exceeds 

 six feet, to place a plano-convex lens, (whose focal 

 length is about equal to a quarter or a fifth of the 

 distance, and whose diameter is not less than one- 

 twelfth of its focal length) with its plain side towards 

 the microscope, so that an image of the sun may be 

 formed about two inches from the diaphragm wheel. 

 This is easily done by placing a card at the dia- 

 phragm wheel to receive the image of the sun. 



The stand, from which the lamp and bull's eye con- 

 denser are removed, is fixed in the horizontal posi- 

 tion, on a firm support, so that the second mirror of 

 the heliostat may be easily made to occupy a position 

 coincident with the optic axis of the microscope. 



The heliostat should be arranged and left working 

 for a quarter of an hour previous to the arrangement 

 of the apparatus for photography. This enables us 

 to test the accuracy of its adjustments, and allows 

 the action of the first wheel full time to be com- 

 municated to that on the mirror. 



These arrangements made, the microscope is fixed 

 in position, the sulphate of copper cell placed be- 

 tween it and the heliostat and everything carried 

 out in exactly the same manner as described when 

 photographing with Swan's incandescent lamp. 



