USE AND MANIPULATION OF MICEOSCOPE 153 



in the focus that has to be effected to secure a sharp photograph. 

 If the fine-adjustment on the microscope is a graduated one, 

 then this is not a matter of such great difficulty ; it is only 

 necessary to determine the number of divisions through which 

 the micrometer screw has to be moved to obtain correct photo- 

 graphic focus, and to alter the fine-adjustment by this amount 

 every time when a photograph is to be taken. It is, of course, 

 assumed that the object is visually focussed beforehand on the 

 ground-glass screen of the camera, and then this allowance 

 made. Obviously this is a somewhat laborious matter, and it 

 is by no means certain that under all conditions the focussing 

 will prove to be as good as might be desired. 



An alternative and far better method is to use invariably 

 a colour- screen with achromatic objectives. This does not 

 very substantially increase the exposure, as it only requires to be 

 of a pale-yellow or pale- green colour one that will effectively 

 cut out the blue and violet rays. Or again, if the screen is 

 approximately monochromatic, whatever colour it may be, 

 the object when focussed visually will nearly always be found in 

 correct focus photographically. The best screens to use for this 

 purpose will be given in Chap. IX of this book, which treats of 

 the use of colour- screens in general. It will be obvious that 

 where possible it is better to obtain apochromatic objectives, 

 since these colour differences do not then arise ; although it 

 must be admitted that where the necessary trouble has been 

 taken to secure a correct colour-screen the difference between 

 the results obtained by using apochromatic or achromatic 

 objectives can often hardly be appreciated. 



It is important to remember that dust on either of the 

 components of the ocular will become painfully evident in the 

 photograph. For this reason it is almost more necessary to 

 ensure that the oculars are kept perfectly free from any dirt 

 than any other optical part. If dust is evident in the field of 

 view, it can at once be determined whether it is on the ocular 

 or in the object, by rotating the ocular itself ; then, if it is on any 

 part of the latter, the image will move round. Steps may then 

 be taken to remove it, but the cleaning should be as carefully 

 performed as in the case of an objective. If it is necessary to 

 unscrew any part, it is essential to see that on replacing it is 

 firmly screwed up again to its correct position. 



