120 ACCESSORY APPARATUS. 



and enlarged to no more than 4 diameters ; whilst, on the other 

 hand, when the tube is drawn out to its whole length, the ob- 

 ject is enlarged 100 diameters. Of course every intermediate 

 range can be obtained, by drawing out the tube more or less ; 

 and the facility with which this can be accomplished, renders 

 such an instrument most useful in various kinds of research, 

 especially those in which it is important, after finding an object 

 with a lower power, to examine it under a higher amplification ; 

 since this may be done, without either a change of objectives, or 

 a transfer of the object to another microscope fitted with a dif- 

 ferent power. It is when the draw-tube is thus made subservient 

 to the use of the Erector, that the value of its rack-and-pinion 

 adjustment is most felt ; for by giving motion to the milled head 

 which acts upon this (Fig. 22) with one hand, whilst the other 

 hand is kept upon the milled head which moves the whole body 

 (it being necessary to shorten the distance between the object 

 and the objective, in proportion as the distance of the image 

 from the objective is increased), the observer — after a little prac- 

 tice in the working together of the two adjustments — may al- 

 most instantaneously alter his power to any amount of amplifica- 

 tion which he may find the object to require, without ever losing 

 a tolerably distinct view of it. This can scarcely be accomplished 

 without the rack movement ; since, if both hands be required to 

 make the alteration of the draw-tube, the readjustment of the 

 focus must be eflTected subsequently. 



45. Micrometer. — Although some have applied their micro- 

 metric apparatus to the stage of the microscope, yet it is to the 

 Eye-piece that it may be most advantageously adapted.' The 

 cobweb micrometer, invented by Ramsden for Telescopes, is pro- 

 bably, when well constructed, the most perfect instrument that 

 the Microscopist can employ. It is made by stretching across 

 the field of a "positive" eye-piece (§ 23) two very delicate paral- 

 lel wires or cobwebs, one of Avhich can be separated from the 

 other by the action of a fine-threaded screw, the head of which 

 is divided at its edge into a convenient number of parts, which 

 successively pass by an index as the milled head is turned. A por- 

 tion of the field of view on one side is cut off at right angles to 

 the cobweb threads, by a scale formed of a thin plate of brass 

 having notches at its edge, whose distance corresponds to that 

 of the threads of the screw, every fifth notch being made deeper 

 than the rest for the sake of ready enumeration. The object 

 being brought into such a position that one of its edges seems to 

 touch the stationary thread, the other thread is moved by the 

 micrometer screw, until it ap])ear to lie in contact with the other 

 edge of the object; the number of entire divisions on the scale 



' The Stage-miorometer constructed by Fraiinhofer is employed by many continental 

 Mioroscopists ; but it is subject to tliis disadvantage, — that any error in its performance 

 is augmented by the whole magnifying power employed ; whilst a like error in the Eye- 

 piece-micrometer is increased by the magnifying power of the eye-piece alone. 



