ON THE USE OF THE MICROSCOPE. 581 



instruments of the German opticians. The whole instrument is based upon a 

 contrivance which enables one to carry the object to be measured through the 

 field of the microscope by a continuous movement in a right line, and to measure 

 the distance performed. A moveable stage is constructed for this purpose, which 

 consists of a plate moveable within grooves. A screw is attached to this plate, 

 by the turning of which it is moved backwards and forwards. This screw is very 

 accurately made of steel, and usually has 100 turns to an inch : such a screw is 

 called a micrometer screw. Each entire turning of the screw, therefore, moves 

 the stage forward at O'Ol". Provided the turning of the screw is perfectly 

 uniform, the stage is moved forwards O'OOOl at each I -100th part of a turn. In 

 order to determine these parts, a disc divided into 100 parts is attached to one 

 end of the screw, and likewise a fixed index, in which the number of the divi- 

 sional parts may be read ; finally, there is also a nonius besides the index, which 

 enables us to determine the 10th part of the 100th part of a turn ; therefore alto- 

 gether 0*00000 1". The measurement by this instrument is effected in the fol- 

 lowing manner. A fine cobweb-thread is fixed across the diaphragm of the 

 eye-piece, and the screw-micrometer being placed upon the stage of the micro- 

 scope, the eye-piece is turned in such a manner that the cobweb crosses the 

 axis of the screw rectangularly. The object to be measured is then laid upon 

 the plate of the micrometer in such a manner that one of its edges exactly touches 

 the thread in the diaphragm ; and the object, by the movement of the screw, is 

 then cautiously carried through the field of vision until the thread touches the 

 other edge of the object. On having accurately observed the position of the di- 

 vided disc at the commencement and end of this operation, the difference of both 

 will exactly give the diameter of the object to the 100,000th part of an inch. It 

 is somewhat difficult during this operation to bring the object exactly into the 

 right position. Some further contrivances are connected with the micrometer, in 

 order to facilitate this. First of all, another plate is laid upon the plate, move- 

 able in the direction of the axis of the screw, which additional plate is also move- 

 able by a screw in a rectangular direction upon the former. A disc is likewise 

 attached to the additional plate, which can be turned exactly round its axis. The 

 placing of the object is thus facilitated. Much controversy has arisen with regard 

 to the advantages of the screw micrometer. Its fault is principally this, that a screw 

 can hardly ever be so accurate as to render its turnings equal among one another, 

 and each single turning uniform in itself. On that account, many have given the 

 preference to the glass micrometer; but this is only owing to a want of knowledge 

 of the manner in which the glass micrometer is manufactured. I have already enu- 

 merated the defects peculiar to this instrument. To these must be added all the 

 faults of the screw micrometer, for the production of a glass micrometer is only pos- 

 vsible by means of a micrometer screw, which forms the lines. In addition to this, 

 there is this disadvantage in the glass micrometer, that it only represents a very 

 small part of the micrometer screw, and, as it may happen, perhaps the most 

 inaccurate one, whilst the screw micrometer, enables us to repeat the measure- 

 ment with different parts of the screw, and therefore it puts us in a position to 

 rectify errors by taking an average of measurements ; and indeed, at the best, 

 there is a limit to the value of these measurements. It only needs intercourse 

 with a practical optician to know the limits of accuracy in these instruments. A 

 single measurement has, therefore, no value at all ; for if we determine with it the 

 breadth of an object at one 10,000th part of an inch, it may in reality be quite as 

 likely to be one 7,000th or one 14,000th. The average, however, from three or four 

 measurements, at different parts of the screw, gives us something like an accu- 

 rate result. But comparative measurements are always the safest for scientific 

 purposes, namely, measuring at the same time, with the same instrument, a well- 

 known, readily attainable object, which is every where of an equal size, for 

 instance, the blood-globules of a certain animal, so that the statement of the size 

 becomes, as it were, the rule to which every one may reduce the results arrived at 

 with his instrument.* 



b. Much depends on the illumination of objects. The more intense the light is 



* For more detailed observations on micrometry, see Quckett's valuable work on the 

 microscope. TRANS. 



p p 3 



