From what has been stated, it is obvious that we mean by an achromatic objective one iu which the usual order of dispersion is so far reversed, that the light, after undergoing the singularly beautiful Felice 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 formulto 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 which, to those who are familiar only with tele scopes, will be quite unprecedented.
The Huyghenean 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. Ilamsden, 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 fie. 2], where L E and F F are the eye- and field-glass, the latter having now its plane face towards the object. The rays from the object are here made to converge at A A, immediately in front of the field-glass, and here also is placed a plane glass on which are engraved divisions of of an inch or less. The markings of these divisions come into focus therefore at the mime timo as the image of the object, and both are distinctly seen toge ther. Thus the measure of the magnified image is given by lucre inspection, and the value of such Immune in reference to the real object may be obtained thus, which, when once obtained, is constant for the same objective. l'lace on the stage of the instrument a divided scale the value of which is known, and, viewing this scale 11.4 the microscopic object, observe how many of the divisions on the scale attached to the eye piece correspond with one of those in the magnified image. If, for instance, ten of those in the eye-piece correspond with one of those in the image, and if the divisions are known to be equal, then the image is ten times larger than the object, and the dimensions of the object are ten times less than indicated by the micrometer. If the divisions on
the micrometer and on the magnified scale were not equal, it becomes a mere rule-of-three sum ; but in general this trouble is taken by the maker of the instrument, who furnishes a table allowing the value of each division of the micrometer for every objective with which it may be used.
on the subject of measuring it may be well to explain the mode of ascertaining the inegnifying power of the compound microecope, which Is generally taken on the assumption before mentioned, that the naked eye bees moat distinctly at the distance of 10 inches.
Place on the stage of the instrtiment, as before, a known divided scale, and when it is distinctly Feel], hold n rule at 10 inches distance from the disengaged eye, so that it may be seen by that eye. overlap ping or lying by side of the magnified picture of the other scale. Then move the rule till one or more of its known divisions corraspond with a number of those in the magnified scale, and a comparison of the two gives the magnifying power.
Having now explained the opt:cal principles of the achromatic com pound microscope, it remains only to describe the mechanical arrange ments for giving thee° principles their full effect. The mechanism of n microscope is of much more importence than might be imagined by those who have not studied the subject.. In the first place, steadi ness, or freedom from vibration, and most particularly freedom from any vibrations which are not equally communicated to the object under examination, and to the lenses by which it is viewed, is a point of the utmost consequence. When, for instance, the body containing the lenses in screwed by its lower extremity to a horizontal arm, we have one of the most vibratory forms conceivable ; it is precisely the form of the inverted pendulum, which in expressly contrived to indi cate otherwise iusennible variations. The tremor necessarily attendant on such an arrangement is magnified by the whole power of the in strument; and as the object on the stage partakes of thin tremor iu a comparatively insensible degree, the image is seen to oscillate so rapidly, as in some cases to be wholly undistinguishable. Such micro scopes cannot possibly be used with high powers in ordinary houses abutting on any paved streets through which carriages are lowing, nor indeed are they adapted to he used in houses in which the ordinary internal sources of shaking exist.