THE VICTORIAN NATURALIST. 105 



necessary that the fine adjustment be of the most perfect type. 

 With the instrument I have here (Watson's University microscope), 

 a motion of i-3o,oooth part of an inch can be imparted by means 

 of the fine adjustment. The microscope should also be fitted 

 with a rack-work substage, and it should be possible to swing 

 the mirror completely out of the way. 



The points I have mentioned are all required if good work is 

 to be done, while many conveniences are not referred to, for the 

 sake of simplicity. 



Then, as regards objectives — there is even more importance 

 attached to these — they should be achromatic, and corrected for 

 the actinic rays. The best microscope objectives are adapted so 

 as to give the finest visual effect, and if these be used for 

 photography it will be found that when the object has apparently 

 been in focus on the ground glass of the camera, the resulting 

 photograph will not be in focus. In the construction of 

 achromatic objectives it is impossible to get rid of all the colours 

 of the spectrum by the conbination of flint and crown glass, owing 

 to the fact that no two substances disperse all the different 

 colours proportionately. The manufacturer has the choice as to 

 which rays shall be corrected so as to be brought to the visual 

 focus of the objective, and generally the rays at the violet end of 

 the spectrum, which exercise the greatest influence on chemical 

 compounds, such as those employed in photography, are not so 

 included, and form an invisible image at a point nearer to the 

 objective than the visual focus. Of course by a little experiment 

 one can soon find out whether his lenses are corrected for photo- 

 graphy or not, and if one could not conveniently get his objective 

 corrected so that the actinic rays should coincide with the visual 

 rays, the difificulty may be got over by the interposition of a weak 

 concave lens (the power needed being ascertained by experiment) 

 between the objective and the ground glass when focussing — 

 removing it when the exposure is to be made. 



As regards eye-pieces, some writers recommend that work 

 should be done without these, on the score that they are intended 

 to project the image on to the human retina, and not on to the flat 

 surface of a photographic plate ; but I have frequently employed 

 them and think the objection suggested is not a practical one, 

 no defect making itself apparent in the completed photograph. 

 The employment of eye-pieces has these advantages: — (i)A 

 shorter bellows camera can be used ; (2) a greatly diminished 

 exposure will suffice ; and consequently (3) a great saving of 

 time is effected. 



About the camera little need be said. The one I have here is 

 admirably suited for the best work. You will notice that it has 

 a rod at the side by means of which the fine adjustment of the 

 microscope can be turned from the back of the camera. It 



