224 MESSRS. R. THRELFALL AND J. A. POLLOCK 



focussing has been accomplished, and the soldering satisfactorily finished, the micro- 

 scope is again mounted and levelled, and the final adjustment made so that the lever 

 upsets when its pointed end rises above the cross wire by one diameter of the point. 

 It generally happens that the focus is not quite correct, but this can now be set right 

 by means of the eye-piece. Finally the eye piece (which is waxed and otherwise 

 fastened to a tube several inches long sliding very tightly inside the outer tube) is 

 finally waxed air tight and firm. All the parts of the microscope are now firmly 

 fixed to the outer tube, and the whole is correctly focussed upon the end of the lever 

 when the latter is in position. It remains to explain how the outer tube of the 

 microscope is prepared. It must be remembered that the outer tube carries the 

 riding level, and that the whole theory rests upon the assumption that the axis of 

 collimation can be brought into an invariable position with respect to the horizontal. 

 In order to avoid spending too much time over levelling the instrument when it is in 

 actual process of observation, it is of some assistance to have the tube of the micro- 

 scope and the V-grooves of the riding level so perfect that, in ordinary phraseology, the 

 level will reverse. The microscope tube was ground and lapped by us on one of 

 BROWN and SHARPE'S cutter grinders, which are not recommended by the makers for 

 producing a cylindrical surface. However, by applying care and attention and 

 rotating the tube between centres we have succeeded in making a tube so cylindrical 

 that we are unable by any of our appliances to detect any defect of form. We have 

 to thank the mechanical assistant of the laboratory, Mr. JAMES COOK, for making the 

 V-grooves so perfect that the sensitive level we employ does in fact reverse when 

 mounted on the microscope tube. 



The riding -level is a very substantial affair, the frame is of brass cut away as much 

 as possible for the sake of lightness. A cross level is attached to the frame by 

 adjusting screws. The main level is mounted in a copper tube (see p. 232). 



Quartz Thread and Lever and Attachments. 



This being the essential part of the instrument, requires considerable care both in 

 its design and its manufacture. The general methods have been described by BOYS,* 

 also THRELFALL ;t but it will suffice for the present purpose to refer the reader to a 

 book on ' Laboratory Arts/J Sections 80 to 91. 



We have already referred to the fact that it is necessary to attend to the 

 dimensions of the thread employed, and will not repeat ourselves except to point 

 out that to arrive at the best relative dimensions as the result of a compromise 

 necessarily implies an immense amount of experimenting. At one time the quartz 



* ' Phil. Mag.,' June, 1887, p. 489. ' Journal of the Society of Arts,' 1889. 'Journal of the Physical 

 Society,' 1894. 'Phil. Mag.,' 1894, vol. 37, p. 463. 

 t ' Phil. Mag.,' July, 1890. 

 \ THRELFALL, Macmillan, 1898. 



