STxVND— STAGE. xiii 



when the camera hieida is used. A brass pin or some similar contrivance should be placed 

 near the joint so as to check the motion of the body of the microscope when it reaches the 

 horizontal position ; no microscope should be without this. 



The microscope should have a coarse "rack and pinion" movement or "quick motion " 

 for adjusting the focus of the lower powers or object-glasses; and when used with an 

 object-glass of about half an inch focus, the image of the object examined whilst coming 

 in and going out of focus, must not appear to move from one side to the other of the field 

 when the body is raised or depressed by the coarse movement. Also when the milled head 

 of the coarse movement is rotated, the motion should feel smooth, not irregular, uneven, 

 or jerking. In some foreign microscopes, the effect of the coarse rack and pinion move- 

 ment is replaced by the sliding of one tube within the other, the body consisting of two 

 tubes working after the manner of those of a telescope. This arrangement is very objection- 

 able, although used by some very good observers, who probably have more tact than most 

 people, and who do not use such high powers as they ought ; for when the highest powers 

 are used it is perfectly intolerable. The objection is somewhat overcome in some micro- 

 scopes by the existence of a fine movement ; but Ave regard the rack and pinion coarse 

 adjustment as essential. 



A "fine movement" or "slow motion" is indispensable, for with the higher powers 

 (one-eighth and one-twelfth) it is impossible to adjust the focus without it. When the 

 finger or fingers are applied to this in its use, no apparent motion of the object must take 

 place ; should this occur, the movement is worthless, unless, at all events, it is veiy shght, 

 and this when tested with the high powers. 



\Vlien the milled head of the fine movement is turned backward and forward, as in use, 

 the motion should be perfectly even, and should be j^roduced very easily, with slight 

 pressure only of the finger or fingers ; moreover, no diff'erence should be distinguishable 

 between the two directions in which it turned, but it should move with equal ease in both. 



The "field" or luminous disc on which the objects viewed through the microscope are 

 apparently delineated, should have its marginal line clear and black. If this line appear 

 coloured, the eyepiece is not as it should be. 



The stage should not be too small (say less than 2\ inches in diameter). To the best 

 instruments a moveable stage is adapted ; but whether this is essential or not is considered 

 a matter of opinion. Undoubtedly with low powers, the moveable stage may be dispensed 

 with, and is not often used ; but with the higher powers its absence is felt greatly, and we 

 should say that it was essentially necessary. In most of the English microscopes, whether 

 provided with a moveable stage or not, there is a " sliding piece " for producing the backward 

 and forward motion of an object, the lateral motion being effected by dnect application of the 

 fingers. If the body of the microscope is to be used in the incUned position, the sliding 

 piece or a moveable stage becomes essential. 



If the moveable stage be present, the " milled heads " should be pretty large, so as to be 

 readily grasped, and a fiat object should remain in focus whilst traversing the field by the 

 movement of the stage. The stage should also be very thin. 



The mirror should have one plane or flat face and another concave. It should not be too 

 small, and its centre should coincide with the axis of the body of the microscope. 



So long as the above conditions are fulfilled, the general form and arrangement of the stand 

 and its parts are of little consequence. It must also be remembered that the complication and 

 accuracy of the apparatus required, will vary according to the kind of investigations pursued ; 

 thus, the structure of the various tissues of animals and that of most plants can be satisfac- 

 torily studied with apparatus which is totally insufficient to display the structure of ceitain of 



