STANDSTAGE. xv 



reagents are to be applied this position is essential ; but when long-continued examination 

 of an object is required, it becomes very painful and fatiguing to keep the head in the 

 position which the perpendicular position of the body requires. Moreover, as in a micro- 

 scope with the joint or arrangement by which the body can be inclined the body can 

 always be placed perpendicularly, the joint is decidedly advantageous. Again, it is almost 

 essential when the camera lucida 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. 



In most microscopes a tube sliding within the body and carrying the eyepiece forms a 

 u draw-tube." By drawing this out the magnifying power becomes enlarged without 

 changing the eyepiece ; it is very useful with the erector or erecting-glass (p. xxii) ; and 

 serves occasionally to produce slight corrections for variations in the thickness of the 

 covers, with immersion-lenses. 



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 objec- 

 tionable, 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 

 microscopes by the existence of a fine movement ; but we regard the rack-and-pinion 

 coarse adjustment as essential. 



A Jine movement or sloiv motion is indispensable j for with the higher powers (one 

 eighth and upwards) 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 very slight, and 

 this when tested with the high powers. 



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

 the motion should be perfectly even, and should be produced very easily, with slight 

 pressure only of the finger or fingers ; moreover no difference should be distinguishable 

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

 both. 



The Jield or luminous disk 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 3 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 is essentially necessary. In most of the English microscopes, whether 

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



