Heliostat in Micro-jphotograjphy . By G. M. Giles. '11 



remedy for this appeared to me to enlarge the disk of light that is 

 thrown on the object. By putting the condenser out of focus one 

 obtains a larger disk, but tiiis at the expense of a great loss of light, 

 besides which circles of varied colours are thus produced of equal 

 variety of actinic power. 



In order therefore to produce a sufficiently large image, I de- 

 termined to make use of a telescoj)ic objective as condenser, instead 

 of the usual short -focussed lenses that are used for this purpose. 



The glass ultimately employed was an achromatic, photographic, 

 single combination of 3^ inches diameter, and about 10 inches focal 

 length. 



The emj)loyment of this condenser necessitated other changea 

 which have resulted in the form of apparatus I am about to 

 describe. 



It consists of a strong base board of wood about 5 feet long by 

 1 foot broad. Erected along one-half of this is a sort of stage, of 

 sufficient beight to raise the centre of the camera to a level with 

 the tube of the microscope when in a horizontal position. 



The camera, which should be of the'" bellows" kind, capable of 

 being drawn out to about 2 feet, and of such a size as to take 

 plates of 6 inches square, is secured to this by means of binding 

 screws. At the oj)posite extremity of the instrument is the mirror ; 

 this is about 4^ inches square, and is mounted, like an ordinary 

 swing-glass, on a circular base-piece about 10 inches in diameter. 

 This circular stand of the mirror is capable of being rotated round 

 a pivot fixed in the base-board, and bas its edge grooved. The 

 part of the pivot that projects above the level of the stand, supports 

 a couple of pulley sheaves. 



On account of the length of tbe instrument, it is necessary 

 that the mirror should be capable of being moved by strings from 

 behind. These are arranged as follows. One endless band passes 

 round the groove in the stand of the mirror, and through sheave.-i 

 let into the back part of the stand for the camera. A second cord 

 by which the mirror is moved on its horizontal axis is arranged 

 thus. One end is fastened to the upper border of the mirror ; it is 

 next passed through one of the sheaves on the pivot ; and is then 

 carried back between the legs of the microscope to a grooved wheel 

 about 2 inches in diameter, provided with a handle, fixed at the 

 back of tbe instrument ; a turn is made round this, and the cord 

 brought back through the remaining sheave on the pivot to be 

 fastened to the lower border of the mirror. 



Between the mirror and the stand for the camera is a sort of 

 tramway, the entire breadth of which should not be more than 

 8 inches. Sliding easily in this is the stand of the long-focussed 

 condenser. This consists of two vertical wooden supports, across 

 the top of which is a transverse piece with an opening into wliich 



