MICROSCOPE. 



MICROSCOPE. 



i ' 



the blue foci B B as much beyond the red foci R R as the turn of the 

 ditu>cM botwern the red and blue foci of the field-lens and eye lent ; 

 u that the wparation B R i* exactly taken up in passing through tbow 

 two l<*n*m, and the whole of the colour* coincide an to focal dUtance aa 

 oon a* the ray* have mawd the eve-lent. Hut while they coincide ai 

 to distance, they differ in another respect ; the blue image* are ren- 

 dered smaller than the red by the superior refractive power of tho 

 Grid glut upon the blue raft. In tracing the pencil L, for instance, it 

 will be noticed that after raining the beld-gloss two set* of lines are 

 drawn, one whole and one dotted, the former representing the red, and 

 the Utter the blue ray. This is the accidental effect in the Huyghoneon 

 eye-piece pointed out by Boocovich. This separation into colour* at 

 the field-glass U like the orer -correction of tho objective ; it leads to a 

 subsequent complete correction. For if the differently coloured rays 

 were kept together till they reached the eye-glow, they would then 

 become coloured, and present coloured images to the eye ; but fortu- 

 nately, and most beautifully, the separation effected by the field-gloss 

 causes the blue rays to fall so much nearer tho centre of the ere glass,, 

 where, owing to the spherical figure, the refractive power is less than 

 at the margin, that the spherical error of the eye-lens constitutes a 

 nearly perfect balance to the chromatic dispersion of the field-lens, and 

 the red and blue rays L' and L* emerge sensibly parallel, presenting, in 

 consequence, the perfect definition of a single point to the eye. The 

 same reasoning is true of the intermediate colours and of the other 

 pencils. 



From what has been stated, it is obvious that we mean by an 

 achromatic objective one in which the usual order of dispersion is so 

 far reversed, that the light, after undergoing the singularly beautiful 

 series of changes effected by the eye-piece, shall come uncoloured to 

 the eye. We can give no specific rules for producing these results. 

 Close study of the formula: for achromatism given by the celebrated 

 mathematicians we have quoted will do much, but the principles must 

 be brought to the test of repeated experiment. Nor will the experi- 

 ments be worth anything unless the curves be most accurately 

 measured and worked, and the lenses centred and adjusted with a 

 degree of precision w Inch, to those who are familiar only with tele- 

 scopes, will be quite unprecedented. 



The Huygheneon eye-piece which we have described is the best for 

 merely optical purposes ; but when it is required to measure the 

 magnified image, we use the eye piece invented by Mr. Uauudcn, and 

 called, from its purpose, the micrometer eye-piece. When it is stated 

 that we sometimes require to measure portions of animal or vegetable 

 matter a hundred times smaller than any divisions than can be arti- 

 ficially made on any measuring instrument, the advantage of applying 

 the scale to the magnified image will be obvious, as compared with the 

 application of engraved or mechanical micrometers to the stage of the 

 instrument. 



The arrangement is shown in fg. 21, where EE and rr are the 



Fig. 51. 



eye- and field-gloss, the latter having now 

 iU plane face towards the object. The 

 rays from the object are hero made to 

 converge at A A, immediately in front of 

 the field-glass, and here also i placed a 

 plane glass on which ore engraved divisions 

 i i of an inch or less. The markings 

 of these divisions come into focus therefore 

 at the same time as the image of the 

 object, and both are distinctly seen toge- 

 ther. Thus the measure of the magnified 

 image is given by mere inspection, and the 

 value of such measures in reference to the 

 real object may be obtained thus, which, 

 when (.nee obtained, is constant for the 

 same objective. Place on the stage of the 

 instrument a divided scale the value of 

 which is known, and, viewing this scale 

 as the microscopic object, observe how 

 many of the divisions on the scale attached 

 to the eyepiece correspond with one of 

 those in the magnified image. If. i-r 

 instance, ten of those in the eyepiece 

 correspond \\i h one of those in the image, 

 and if the divisions are known to be 

 ' <|ii il. then the image U ten times larger 

 than the object, and the dimensions of the 

 object are ten times less than indicated 

 by the iiiii-io ..'!. If the divisions on 

 the micrometer and on the magnified scale 

 were not equal, H become* a mere rule of-thrcc sum : but in general 

 this trouble U token by the maker of '!,- . who furnishes a 



table showing the value of each division of the mi 

 objective with which it may be used. 



While en the uliji-ct of measuring it may ) well to explain the 

 mod* of ascertaining the magnifying power of the compound 

 rnicnwc'ip", which is generally t the- assumption before 



mentioned, that the naked eye sees most distinctly at the distance 

 of 10 inches. 



Place on the stage of tlw instrument, u before, a known divided 



scale, and when it is distinctly seen, hold a rule at 10 inches distance 

 IV 'in the disengaged eye, so that it may be seen by that 

 ping or lyiiitf by side of the magnified pi< turo of the other noa!c. Then 

 ni"\e the rule till one or more of its known divisions correspond with 

 a number of those in the magnified scale, and a comparison of the 

 two gives the magnifying power. 



'.i.niiatic com- 

 pound microscope, it remains only to describe t i -al arrsugc- 

 ments for giving those principle* their full effect. The n . 

 a inicrosco| is of much iiu>re importance than might be imagined by 

 those who liave not studied the subject In the firut place, steadi- 

 ness, or freedom from vibration, and most particularly freei loin from 

 any vibrations which are not equally communicated to the object 

 iiinler examination, and to the lenses by which it is viewed, is a point 

 of the utmost consequence. When, for instance, the body contouring 

 the lenses is screwed by its lower extremity to a horizontal ami, we 

 have one of the most vibratory forms conceivable; it is precisely the 

 form of the inverted pendulum, which is expressly contrived to indi- 

 cate otherwise insensible variations. The tremor necessarily attend nit 

 on such an arrangement is magnified by the whole power of the in- 

 -iriuiieiit; and as the object on the stage partakes of this tremor in a 

 comparatively insensible degree, the image is seen to oscillate so 

 rapidly, as in some cases to be wholly undistinguislialile. Such micro 

 scopes cannot possibly be used with high powers in ordinary houses 

 abutting on any paved streets through which carriages are pawing, nor 

 indeed are they adapted to be used in houses in which the ordin uy 

 internal sources of shaking exist. 



One of the best modes of mounting a compound microscope is 

 li"v.n in the annexed view (ji'j. 22). which, though too small to 



Fig. 22. How's Microscope. 



exhibit all the details, will servo to explain the chief features of the 

 arrangement. 



A A are two uprights surmounting the tripod base, B B B. At c is 

 the axis u[K>n which the instrument turns, and by which any inclina- 

 tion, from vertical to horizontal, maybe given to it, in which position 

 it may IK' clamped by the small handle D when the use of over-balanc- 

 ing apparatus on the secondary stage renders this necessary. Motion is 

 :-! n to the bar R by the milled head P : a corresponding milled head 

 on tho other side of the instrument may also be used for the, same pur- 

 pose. At the end of the arm o o in fixed the coin|x>und tube HOI, 

 which receives at it* upper extremity the eye-piece J : at its lower 



