152 PBACTICAL PHOTO-MICEOGEAPHY 



For observational work the selection of the most suitable 

 ocular, and the advisability of using low- or high-power ones, is 

 an important matter ; but in photo-micrography any but low- 

 power oculars may, except under very special circumstances, 

 be excluded. On occasions it may be necessary, where great 

 amplification is required, to use one of fairly high power ; 

 but this will only be found to work satisfactorily when the 

 objectives are apochromats, and the oculars therefore of the 

 compensating type. If an ocular other than of the projection 

 type is used, no adjustment of its parts is necessary when a 

 photograph is being taken, but with the projection- oculars 

 it is important to see that the image of the diaphragm inside 

 the ocular itself is quite sharply projected on the focussing- 

 screen of the camera. This procedure has been sufficiently 

 described under the section giving the various types of oculars. 

 It is essential to ensure that, in the case where no ocular is 

 used in conjunction with the objective, the inside of the body- 

 tube of the microscope is completely blackened, as otherwise 

 internal reflections are sure to result, and a central flare-spot 

 will appear on the focussing- screen of the camera. This is 

 most effectively provided against by having a little cardboard 

 tube which is lined with black velvet, and which is of such a 

 size that it slips into the body-tube; it should be of such a 

 length that it comes quite flush with the top of the latter. 



Achromatic objectives that may give an extremely good 

 visual image are often not corrected to give a sharp photograph 

 in the same plane ; in fact, generally the better the visual 

 image, the less likely is it that the photograph will come out 

 sharp. The reason of this is that achromatic lenses are corrected 

 for the yellow and green rays only that is, for the portion of 

 the spectrum of the greatest visual luminosity ; but they are 

 not corrected to utilise in the formation of the image the less 

 luminous blue and violet rays. With apochromatic objectives 

 this does not apply, and it will be found that these give a 

 perfectly defined image whatever the illumination may be. 

 One method of overcoming the difficulty with achromatic 

 objectives is to ascertain the difference between the distances 

 of the visual focus and the actinic focus from the microscope 

 stage. This can only be effected by taking a series of photo- 

 graphs with each lens, and carefully measuring the alteration 



