264 now TO WORK 



and the achromatic condenser placed in the fitting on the under- 

 surfacc of the stage-plate. The blackened card diaphragm, according 

 to the size of the field desired, is to be fixed in the diaphragm frame 

 that works to and fro in the cut in the back part of the camera 

 chamber, and the prism so turned that the sunlight is thrown on the 

 ground-glass screen. Then bring the objective into focus as when the 

 object slide is in situ. The value of the prism is now apparent, for npon 

 standing with the face towards its convex surface, and turning it on its 

 own parallactic motion, an intense image of the sun will be soon found, 

 as it were, on that surface : the prism is then to be so arranged that 

 the reflected images from the lens or lenses of the achromatic con- 

 denser and of the objective, fall centrally on the sun's image. If the 

 field on the ground glass now appears equally bright in all directions, 

 the achromatic condenser is slightly altered, to see whether any 

 increase of illumination accompany the change; if not, it is returned 

 to its previous position. Should the images not all fall into the line 

 of the image of the sun, seen on the surface of the prism, some 

 alteration must be made in the part which seems most at fault ; but 

 when they all fall into it, and the distance of the prism is such that 

 its converging rays just cross before reaching the object, the proba- 

 bilities are that the centering is correct. If the prism will not carry 

 a cone of light sufficiently large and bright for the lowest powers, as 

 3 inches, then set it aside and try the plane mirror. The object, if 

 on the ordinary 3x1 inch slide, is now placed on the stage, the 

 camera bellows-body shut up ; the whole apparatus is covered with a 

 large focussing cloth of black cotton velvet, except the parts to be 

 exposed to the light, the right hand is applied to the slide, and the 

 eyes directed to the ground-glass screen under the focussing cloth ; the 

 object is now placed (as nearly as possible) in the centre of the field, 

 and the approximate adjustment made. Alter the rack-work of the 

 condenser and the prism until the best effect is produced, for the 

 proper position of the condenser is a very important one, and often 

 more trouble to arrange than the focus of the object-glass. The 

 object being well centered, the field perfectly bright and uniform, see 

 that the velvet collar around the microscope tube abuts closely against 

 the aperture in the door of the vertical frame ; now withdraw the 

 camera along the base-board from the near end, and closely watch the 

 enlargement ; when this is determined on, fix the camera by the wire 

 pins to the nearest hole in the two wooden guides. Supposing this 

 to be at an easy working distance, watch the image on the ground 

 glass through the focussing eye-piece ; turn the graduated milled- 

 headed screw of the fine motion until the same point as was previously 

 noted is brought into a sharp focus. Should the over-correction of 



