214 



MICROMETER. 



ters 



Doubly Ingenious as these micrometers are, we conceive them 

 Refracting to be liable to an objection of a very serious nature, 

 rome- j-j r _ u rews ter has shewn, from numerous experiments, 

 that rock crystal is very imperfectly crystallized in the 

 direction of its axis, and that it exhibits great devia- 

 tions from a homogeneous texture, both in its action 

 upon common and polarised light. Distinct images 

 therefore cannot be formed through prisms or lenses 

 whose refracting surfaces are perpendicular, or nearly 

 so to the axis of the hexaedral prism. If astronomy 

 therefore is to derive any advantage from doubly re- 

 fracting micrometers, we must have recourse to the 

 double refraction of other substances than rock crys- 

 tal. 



CHAP. IX. 



On Micrometers for Microscopes. 



Dr. Wollas- 

 ton's single 

 lens micro- 

 meter. 



On micro- Microscopical micrometers are generally employed 

 meters for for measuring very minute spaces, such as the subdi- 

 micros- visions of astronomical instruments, or the diameters 

 copes. o f objects magnified in the field of a microscope. All 



the micrometers described in chapter first, may be 

 considered as micrometers of this kind, when placed 

 in the anterior focus of the eye-glass of a micro- 

 scope. 



One of the simplest micrometers for microscopes, 

 consists of a screw with an index, fixed in the focus of 

 the eve-glass. The point of the screw is made to move 

 across the space to be measured; and as the index re- 

 gisters the number of revolutions necessary for this 

 purpose, the diameter of the object will be found by 

 ascertaining the number of threads in one inch. 



Dr. Jurin measured the magnitudes of minute ob- 

 jects by comparing them with small pieces of silver or 

 brass wire placed beside them ; the diameter of which 

 had been previously ascertained by coiling it round a 

 cylinder, and observing how many breadths of the 

 wire were contained in a given number of inches. 



One of the best microscopical micrometers is the 

 single lens micrometer, invented by Dr. Wollaston. 

 This instrument, says Dr. Wollaston, is furnished with 

 a single lens of about -r^th of an inch focal length. 

 The aperture of such a lens is necessarily small, so 

 that when it is mounted on a plate of brass, a small 

 perforation can be made by the side of it in the brass, 

 as near to its centre as T ] r tn f an inch. 



When a lens thus mounted is placed before the eye 

 for the purpose of examining any small object, the 

 pupil is of sufficient magnitude for seeing distant ob- 

 jects at the same time through the adjacent perfora- 

 tion, so that the apparent dimensions of the magnified 

 image might be compared with a scale of inches, 

 feet, and yards, according to the distance at which it 

 might be convenient to place it. 



A scale of smaller dimensions attached to the in- 

 strument will, however, be found preferable on ac- 

 count of the steadiness with which the comparison 

 may be made ; and it may be seen with sufficient dis- 

 tinctness by the naked eye, without any effort of nice 

 adaptation, by reason of the smallness of the hole 

 through which it is viewed. 



I'IATE The construction that I have chosen for the scale is 



rccuivi. represented in Plate CCCLXXVI. Fig. 21. It is corn- 

 Fig. 21. posed of small wires about r ' s th of an inch in diame- 

 ter, placed side by side, so as to form a scale of equal 

 parts, which may be with ease counted by means of a 

 certain regular variation of the lengths of the wires. 



The external appearance of the whole instrument is On Micro- 

 that of a common telescope consisting of three tubes. meter for 

 The scale occupies the place of the object-glass, and micr - 

 the little lens is situated at the smaller end, with a ^P es ^ 

 pair of plain glasses sliding before it, between -which 

 the subject of examination is to be included. This 

 part of the apparatus is shewn separately in Fig. 23. Fig. 23. 

 It has a projection, with a perforation, through which 

 a pin is inserted to connect it with a screw represent- 

 ed at b, Fig. 22. This screw gives lateral motion to Fig. 28. 

 the object, so as to make it correspond with any parti- 

 cular part of the scale. The lens has also a small mo- 

 tion of adjustment, by means of the cap c, Fig. 22. 

 which renders the view of the magnified object dis- 

 tinct. 



Before the instrument is completed, it is necessary 

 to determine with precision the indications of the 

 scale, which must be different according to the dis- 

 tance to which the tube is drawn out. In my instru- 

 ment, one division of the scale corresponds to To-J^s*' 1 

 of an inch, when it is at the distance of 16.6 inches 

 from the lens; and since the apparent magnitude in 

 small angles varies in the simple inverse ratio of this 

 distance, each division of the same scale will corres- 

 pond to T^^th, at the distance of 8-f 3 inches, and the 

 intermediate fractions ^Vs. T 3-V?r> &c - are found by 

 intervals of 1.66 inch, marked on the outside of the 

 tube. The basis on which these indications were 

 founded in this instrument, was a wire, carefully as- 

 certained to be T ig. of an inch in diameter, the magni- 

 fied image of which occupied fifty divisions of the scale, 

 when it was at the distance of 16.6 inches ; and hence 



one division = = 7*' ^' nce an y error ' ft 



the original estimate of this wire must pervade all sub- 

 sequent measures derived from it, the substance em- 

 ployed was pure gold, drawn till fifty two inches in 

 length weighed exactly five grains. If we assume the 

 specific gravity of gold to be 1 9.36, a cylindrical inch 

 will weigh 3837 grains ; and we may thence infer the 

 diameter of such a wire to be T J^ of an inch, more 

 nearly than can be ascertained by any other method. 



For the sake of rendering the scale more accurate, 

 a similar method was in fact pursued with several gold 

 wires of 'different sizes, weighed with equal care; and 

 the subdivisions of the exterior scale were made to cor- 

 respond with the average of their indications. 



In making use of this micrometer for taking the 

 measure of any object, it would be sufficient, at any 

 one accidental position of the tube, to note the num- 

 ber on the outside as denominator, and to observe the 

 number of divisions and decimal parts which the sub- 

 ject of examination occupies on the interior scale as 

 numerator of a fraction, expressing its dimensions in 

 proportional parts of an inch ; but it is preferable to 

 obtain an integer as numerator by sliding the tube in- 

 ward or outward, till the image of the wire is seen 

 to correspond with some exact number of divisions, 

 not only for the sake of greater simplicity in the arith- 

 metical computations, but because we can by the eye 

 judge more correctly of actual coincidence than of the 

 comparative magnitudes of adjacent intervals. The 

 smallest quantity which the graduations of this instru- 

 ment profess to measure, is less than the eye can really 

 appreciate in sliding the tube inward or outward. If, 

 for instance, the object measured be really 7^5-, it may 

 appear T53trs> or WTO* * n which case the doubt amounts 

 to T \j. part of the whole quantity. But the difference 

 is here exceedingly small in comparison to the extreme 

 division of other instruments where the nominal extent 



