2128 Microscopical Society* 



November 24, 1847. — J. S. Bowerbank, Esq., F.K.S., President, in the chair. 



The minutes of the preceding meeting were read and confirmed. 



A Report of three papers ' On the Structure of the Teeth,' by A. Nasmyth, Esq., 

 was presented by Mr. Legg, and the thanks of the Society voted to him for this 

 present. 



A certificate in favour of George Arthur Knighley Howman, Esq., 34, Brompton 

 Square, was read, and ordered to be suspended in the meeting-room. 



Charles Sterry and W. S. Gillett, Esqrs., were balloted for, and duly admitted 

 members of the Society. 



A paper by George Jackson, Esq., ' On Micrometers,' was read. After some re- 

 marks on the necessity of having some mode of accurately ascertaining the size of 

 minute objects subjected to microscopical examination, and also on the means gene- 

 rally employed, together with an explanation of the method of ascertaining the value 

 of the divisions of an eye-piece micrometer with a particular object-glass, and also a 

 comparison of the relative advantages of the positive or negative eye-piece in observing 

 ruled glass micrometers. The author proceeded to describe the micrometer which he 

 had constructed, and which was laid before the meeting. Short bold lines are ruled 

 on a piece of glass, and, to facilitate counting, every fifth line is drawn longer, and 

 every tenth still longer, as in the common rule. Very finely levigated plumbago is 

 rubbed into the lines, to render them visible, and they are covered by a piece of thin 

 glass, cemented by Canada balsam, to prevent the plumbago from being rubbed out. 

 The slip of glass thus prepared is placed in a thin brass frame, so that it may slide 

 freely, and is acted upon at one end by a pushing-screw and at the other by a slight 

 spring. Slits are cut in the negative eye-piece on each side, so that the brass frame 

 may be passed along the field in the focus of the eye-glass, which should also allow 

 of adjustment for different eyes. The brass frame is retained in its place by a spring- 

 within the tube of the eye-piece, the object is brought to the centre of the field by the 

 stage movements, and the coincidence between one side of it and one of the long lines 

 is made with great accuracy by means of the small pushing-screw. The divisions are 

 then read off as easily as the inches and tenths in a common rule. The author con- 

 cluded by pointing out the best methods of ascertaining the value of the divisions of 

 the micrometer, and with some plain directions for its use. 



Another paper, being ' A Description of certain Arrangements for effecting the 

 Measurement of Microscopic Objects, with some Observations relative to the Ex- 

 pression of Minute Dimensions,' by George Shadbolt, Jun., Esq., was also read. The 

 principle adopted by Mr. Shadbolt was stated by him to be that of viewing the object 

 and the micrometer with the same power, using a contrivance for keeping the size of 

 the former in view while inspecting the latter. This is effected by means of moveable 

 calliper points, placed in the focus of the eye-glass of the eye-piece, which points are 

 adjusted by a couple of screws with milled heads, projecting on each side of the eye- 

 piece. He gave his reason for preferring points to parallel lines, — considering the 

 measurement from point to point to be capable of much greater accuracy than that 

 from one parallel line to another, in consequence of the difficulty of getting the object 

 exactly at right angles to both lines. He then described the mode of constructing his 

 micrometer. A disc of metal is to be made quite flat on one side, and is to be of the 

 same diameter as the diaphragm plate, with a similar central aperture, a trille smaller. 

 This disc is also to have two slots cut in it, parallel one to the other, one on each side 

 of the central opening, rather longer than its diameter. The disc is to be attached by 



