MEDIUM-POWER PHOTO-MICROGHAPHY 



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objective central also. It is very obvious that the lenses will now perform at their 

 best, for every object will be axially viewed by each objective. 



The Davis Diaphragm. — ^Before quitting the subject of lenses, we must 

 mention, as perhaps the most suitable place for so doing, that it is very useful to 

 have placed in the microscope tube, just above the objective, an auxiliary iris 

 diaphragm. It has been called the Davis diaphragm. Its object in this position is 

 to very slightly curtail the marginal rays of low-power objectives, by doing which 

 even with apochromatics a better definition — one full of greater contrast — is often 

 obtained. Any reflections too from the side of the tube may be cut off by this 

 means. It should be made truly central with the axis of the microscope and should 

 be in no wise closed when using lenses of high aperture ; for if such should be done — 

 seeing that resolution entirely depends upon the numerical aperture of the lens, as 

 already hinted — the wide -angled objective becomes a narrow-angled one and will cease 

 to possess its separating power. 



(iv) The Mechanical Stage. — It is a great convenience, indeed almost a neces- 

 sity, for the microscope to have a mechanical stage. It must be well made and possess 

 verniers, so that when objects are once found and noted they can be brought again 

 into the field of view without any trouble. It is better for the stage to be capable of 

 rotation about the axis of the microscope, and such motion is a great convenience, for 

 at times specimens are not placed upon the slide in a position favourable to photograph 

 them without they are turned on their axes to the right or left. If no rotation of the 

 stage is possible, the object itself has to be moved, which, when employing a high- 

 power, frequently causes it to pass out of the field of view, thus necessitating time 

 and trouble to find it again. Should, however, the expense involved in having a 

 rotating stage be thought undesirable, still a mechanical one should always be obtained. 

 Even this is costly, but that can be got over by employing one of the many forms of 

 removable stages that are sold by most opticians which are not so costly. To a great 

 extent these are all made after the Mayall model, which has one rack and one endless 

 screw. None of these last so well as the ordinary type of permanent mechanical stage 

 as met with in the best microscopes, for the endless screw soon shows sign of wear, 

 and the loss of way rapidly becomes very annoying. To obviate that Messrs. Baker, 

 of Holborn, have quite recently brought out a new model which only requires two 

 screws to attach it to any ordinary stage. We have examined it carefully and can 

 recommend it, as it seems likely to wear well, having no endless screw in its con- 

 struction, and besides, it allows considerable movement in both directions, each of 



