PRACTICAL PHOTO-MICROGRAPHY. 85 



Now come in the skill and experience of the worker, for the 

 objective has now to be corrected for the thickness of the 

 cover glass and the position of the test hairs with regard to the 

 cover glass. The correction is to be accomplished by means 

 of " screw collar," if the objective has one, or by length of 

 tube, if the objective is without collar. We cannot instruct 

 in this matter. The screw collar or the length of tube is to be 

 altered gradually till the hair under observation is shown as 

 long as it can be shown, as black also and as finely pointed. 

 In critical image work it is better to choose one hair and con- 

 fine the attention to that hair ; if the object be not flat, of 

 course other hairs will not be equally sharp, but we have noth- 

 ing to do with that ; our business is to get a perfect image of 

 our one object. If we require the best general photo-micro- 

 graph of our " tongue " we must set about it by method No. 2. 

 The objective being corrected, and the image perfect as seen 

 with the ordinary eye-piece, the latter is removed and the pro- 

 jection ocular substituted ; the image is then projected first 

 upon the ground glass of the camera. Assuming that a pro- 

 jection ocular of the type made by Zeiss is to be used, we have 

 next to focus the diaphragm of this ocular upon our ground 

 glass, where a round disc due to the diaphragm in the ocular 

 will be seen.* This is easily done by observation, twisting 

 round the moveable part of the ocular, where we shall find 9 

 scale and an index put there for the purpose. Next the image, 

 having been carefully centred on the ground glass, is accurately- 

 focused on the plain glass prepared as suggested on page 53. 

 Here again we can not instruct ; it is a matter of experience 

 to focus a difficult image. With objectives, other than apo- 

 chromatic, fringes of color are usually found round the critical 

 part of the image ; very often, with ordinary objectives, the 

 focus will be found correct when the colors seen are claret and 

 green ; but this depends on the correction for color given to 

 the lens by the optician. The apochromatic lenses, in our 



* If the image of the diaphragm as projected is larger, we may have to 

 slew the camera a little in order to see the edge of the projected disc 

 image. 



