210 PRACTICAL MICROSCOPY. 



and the instrument provided with a coarse and fine adjust- 

 ment, the latter answering the helm when turned either 

 backwards or forwards ; but more particularly must it be 

 sensitive to very slight movements of the milled head when 

 withdrawing the objective from the stage. 



Dr. Woodward states " the objective selected should 

 always be specially corrected for photography." The 

 author is not able to agree with Dr. Woodward on this 

 point ; no doubt it is very convenient to have one so 

 corrected, but it is by no means imperative. To those as 

 yet uninitiated in the mysteries of the photographic art, it 

 may be necessary to say that the visual and actinic foci 

 often (though not necessarily) lie in different planes, and 

 therefore an objective which gives perfect definition to the 

 eye will often produce a blurred and indistinct image upon 

 the sensitive plate. This may be remedied in withdrawing 

 the objective, by turning the milled head of the fine adjust- 

 ment ; it cannot be told beforehand what allowance is 

 necessary, the only method is by trial and error; some 

 objectives require no correction whatever, while others 

 which necessitate considerable movement from the object, 

 can be made to yield just as perfect results. 



The best way to arrive at the necessary correction is to 

 take a picture at the best visual focus, develop, fix, and dry 

 in the usual manner, and then withdraw the objective from 

 the stage by means of the fine adjustment until the image 

 appears on the ground glass about as indistinct as it does 

 in the negative. This will probably be the amount of cor- 

 rection required, and one or two trial plates will decide the 

 question. For powers up to the half-inch the proboscis of 

 the fly (Fig. 99) is a good object to experiment with ; 

 a photograph of this preparation, taken with a Ross's 

 ij-inch, showed that its actinic focus was identical with 



