PHOTOMICROGRAPHIC APPARATUS 11 



out. Tyler's micro-grain screens are very suitable, and 

 they usually have a clear glass centre. If there is no clear 

 space on the ground glass, a cover slip should be cemented 

 with Canada balsam on to its centre, as it is so much 

 easier to focus fine detail accurately on clear glass than 

 on ground glass, however fine. A focussing lens is used 

 to see the image under these conditions, and it is neces- 

 sary to make sure that it is focussed on the glass and not 

 on an aerial image in another plane. For this purpose a 

 scratch is made on the clear glass, or lines drawn across 

 the ground glass before the cover glass is cemented on, 

 and these are kept in focus at the same time as the image. 

 Some workers prefer a clear glass screen which can be 

 substituted for the ground glass for final focussing, after 

 the preparation has been searched and properly centred 

 on the latter. Lines are ruled diagonally across the ground 

 glass to aid in centring the light or the image, and it is 

 also advisable to rule rectangles of lantern-slide size, and 

 of the sizes of all the plates that may be used, to 

 coincide with the positions of the adapters in the dark 

 slides. 



Centring the Apparatus. The greatest care must be 

 taken when making the apparatus to ensure that the 

 optical axes of all the parts are truly in line ; if this is 

 done at the outset all the components can be placed 

 very rapidly in their proper positions, and only very 

 small adjustments of the lamp or of the condensers, by 

 means of their centring screws, are necessary before 

 taking a photomicrograph. The most careful measure- 

 ments are hardly sufficiently accurate for this purpose, 

 and the directions given on page 145 for centring the 

 microscope, lamp, and condensing lenses should be 

 followed out before their positions are definitely 

 fixed. 



Watson's Laboratory Camera. The two cameras just 

 described were made without any expert assistance, 



