LOW-POWER WOEK 21 



further from the condenser, the tray of the jet being provided with a groove to enable 

 such movement to be done with ease and regularity. 



We next look at the ground glass of the camera, which is placed at P (the lens 

 being, of course, screwed in its place at L), and adjust the specimen until it is central. 

 Drawing the head back some lo or 12 inches will enable the whole field of view to be 

 better scrutinised than it would be if the head were placed nearer. If it is seen that 

 one side is brighter than the other, either the lime jet itself must be pushed a trifle 

 from side to side on the supporting pin provided with the appara.tus, or, what is 

 better, the whole limelight box with its condenser should be bodily shifted from side 

 to side. If the light be unequal at the top or the bottom the lime must be raised or 

 lowered as occasion requires. The greatest care should be exercised in getting the 

 illumination equal, and it requires some amount of practice to be able to do it. It 

 repays the trouble, however, for it is exceedingly provoking after taking what might 

 be otherwise a good photograph, to find that a portion of it is not so bright as the 

 rest, as shown in Fig. 6, Plate I. There is another reason, too, why the light should 

 be carefully centred, for even in this low-power work perfect definition certainly 

 depends in a measure upon it. 



Assuming then we have got over this trouble, we now examine our specimen upon 

 the support S, seeing that it is firmly clamped by the spring clips already mentioned. 

 We then push the extended camera quite close to the object — say when using the 

 50 mm. lens to about an inch ofi" it. It will be now necessary to push the bellows to 

 and fro till we get the image on the screen. If such image be too large or too small 

 it is very obvious the camera must be removed further ofi" or brought nearer to the 

 object, the bellows being readjusted to obtain the rough focus on the ground glass 

 screen. Some little practice is required in this matter because the exact length of 

 camera and the exact distance of the lens from the object are the factors for producing 

 a definite amount of enlargement. It may be necessary with small objects, when we 

 want 10 diameters magnification, to add the lengthening piece of the camera to which 

 reference has already been made ; in that case the lens may have to be pushed up 

 almost in contact with the object. When the image is clearly seen and roughly 

 focused by moving the ground glass it should always be measured, so as to enable 

 the photographer to see whether the magnification is such as he desires. This should 

 never be omitted, for it is often of great service at some future date to be able to refer 

 to the number of diameters an object has been magnified ; but besides this, it 

 introduces an element of accuracy which, if the photo-micrographer does not already 

 possess, he must rapidly learn as soon as possible. When taking negatives for the 



