80 



THE MICROSCOPE 



Table ot 



drawtube 



corrections. 



< 



1 The lower power lenses selected for the purpose of compiling this 

 table are not as usually supplied, but are specially corrected for longer 

 tubes to show the variation better. 



Canada balsam acts in a similar manner to glass, and a layer 

 of Canada balsam between the cover glass and the object has the 

 efEect of increasing the thickness of the cover glass. It must 

 be allowed for unless the object is mounted in contact with the 

 under-surface of the cover glass. , 

 Correction of It will occur to the reader that the correct tube length can 

 tototaeffi'by be ascertained by observing the image through the microscope. 

 obserration. ipjiig jg ygry readily done under dark-ground illumination, and 

 with more difficidty with transmitted light. Under dark- 

 ground illumination there wiU always be fine specks of dust 

 illuminated as brilliant points ; one which is so small as to show 

 no apparent outline or shape should be selected and be placed 

 near the centre of the field of view. It should then he focussed 

 backwards and forwards, and the image examined on each side 

 of the sharpest focus. If the light is aU coming to an exact focus 

 at any one point, the appearance on each side of the focus will be 

 the same : it will be a small disc or patch of light equally intense 



on either side. If it is out of adjustment, one side wiU be clear 

 and the other side hazy. Fig. 89 shows that if aU the light 



