268 



SCIENCE-GOSSIP. 



[Dec. 1, 1865. 



makes it a very difficult matter to obtain the focus 

 of a good objective. And it not unfrequeutly gives 

 an apparent motion to tlie object on the stage, which 

 may be seen gliding backwards and forwards to the 

 extent of an inch or more over the field. Such a 

 microscope, however cheap it may be, should be 

 studiously avoided, as it will inevitably prove a 

 source of constant irritation to its possessor. In 

 some of the cheapest instruments, the rack-work and 

 pinion movement is dispensed witli altogether. The 

 focussing of the lenses is accomplished by sliding the 

 body up and down an outer tube which embraces 

 and supports it. A microscope of this character 

 may be used with low powers, but it can never be 

 made to work with»the same degree of accuracy as 

 those which possess the higher mechanical move- 

 ments. There is one other mode of adjustment 

 which I think it right to mention here. It is known 

 as the chain movement. A watch chain is in this 

 case substituted for the rack-work. The chain is 

 wound round a small drum, connecLed with the 

 usual milled heads. The movement thus obtained 

 is so beautifully smooth and so little likely to get 

 uneven by wear, besides obviating the necessity of 

 a fine adjustment, that I cannot understand why it 

 is not more universally adopted. The only objection 

 I have heard urged against it, is that the chain, being 

 made up of so many small links and rivets, is, like 

 a fortress, no stronger than its weakest point : it is 

 liable to snap asunder. The old chain may, however, 

 be easily replaced by a new one, at the cost of a 

 few shillings. Besides the movements already 

 alluded to, and known as the coarse adjustment, there 

 is in all the better class of microscopes a much more 

 delicate mechanical contrivance termed the fine 

 aclj?ishient, which, as its name implies, gives greater 

 facilities for securing to the greatest nicety the focus 

 of a one-eighth or one-twelfth objective. In the 

 former arrangement the entire body or tube is moved 

 up or down by the rack-work ; in the latter, it is the 

 object-glass alone that is affected. This advantage 

 is secured by attaching the object-glass to a short 

 piece of tubing that fits accurately the interior of 

 the lower part of the body, and at the same time 

 slides freely in it. The tube is kept down in its 

 place by a spiral spring, which presses upon it from 

 behind. Motion is communicated to the tube by a 

 graduated milled-headed screw, acting on the end of 

 a lever. The screw itself contains 150 threads to 

 an inch. One revolution, therefore, of its thread 

 raises or depresses the tube, carrying the object- 

 glass the 150Lh of an inch. The tenth part of a 

 revolution gives us the 1,500th, while the half of a 

 division realizes a movement not exceeding the 

 3,000th of an inch. This extreme nicety of adjust- 

 ment may, to the beginner, seem unnecessary, but 

 to the practised operator it is found to be not a bit 

 too sensitive for his minute investigations. It also 

 proves a safeguard for the object-glass, when, through 



inadvertence, it is brought in contact with the glass 

 that covers an object. The spring tube yields to the 

 pressure, carrying with it the objective, which is thus 

 sometimes saved from destruction. 



We now come to the stage : this, iu its simplest 

 form, consists of a flat brass plate, with a circular 

 hole in its centre for the transmission of light 

 from the mirror beneath it. It should be broad 

 and roomy, and firmly attached to the bar that 

 supports it ; not too thick or deep for oblique 

 illumination, nor so thin as to be springy under a 

 slight pressure of the manipulator's hand. Such 

 a stage is furnished with a sliding ledge, against 

 which objects rest when the instrument is used iu 

 an inclined position; but there is no mechanical 

 ariangement for giving motion to an object; this 

 must be effected by the fingers of the observer. It 

 is not, however, every microscopist who possesses a 

 touch delicate enough for such a purpose. Although 

 stage movements necessarily render a microscope 

 more expensive, the advantage more than counter- 

 balances the extra cost. A moveable stage consists 

 of two or three flat plates, sliding one over the other 

 iu rectangular directions, by means of screw and 

 rack-work. Such movements should be as perfect 

 as those required for the body of the instrument. 

 Tlic value of a stage with such mechanical appliance 

 is best appreciated when high powers are employed. 



By turning the milled heads on either side of the 

 stage, the stage-plate carrying the object slowly 

 traverses the field either in the direction of its 

 length or breadth, and the whole of an object too 

 large to be seen in the field at one view is thus made 

 to pass slowly before the eye. If both heads are 

 turned at the same time, the object then takes a 

 diagonal direction. In the best microscopes there is 

 also a circular motion j)roduced by rack and pinion, 

 which enables the stage-plate to revolve three-fourths 

 of a circle, or, in come cases, to make a whole revo- 

 lution. Such an arrangement is advantageous, 

 provided it be not obtained by sacrificing other 

 conveniences of more importance. The ordinary 

 traversing motion of a stage should not be less than 

 three-quarters of an inch in each direction; if ex- 

 tended to an inch, or even more, so much the better 

 for the operator, who will be saved the trouble of 

 shifting his specimen. There is yet another con- 

 trivance, known as White's Lever Stage, in which 

 motion is communicated to the stage by a lever. By 

 applying a slight force to its handle or free end, the 

 stage-plates ghde quickly one upon another, thereby 

 presenting considerable facilities for following the 

 rapid evolutions of the numerous agile animals found 

 in stagnant water. We had almost forgotten another 

 useful and simple arrangement — the magnetic stage, 

 which consists of a single plate of brass, having two 

 narrow tongues projecting slightly above its surface, 

 and running longitudinally at equal distances on each 

 side of the circular opening. The tongues being iu 



