Mr. F. D. Brown's Notes on Thermometry. 59 



A mahogany board, B B (PI. II. fig. 1), about 18 inches long 

 and 4 inches wide, is provided with a groove, Gr Gr, of the shape 

 shown in the section (fig. la); a piece of gun -metal, about 

 5 inches long and \ inch thick, slides in this groove with some 

 little difficulty — the friction, which is produced by the spring 

 ffj being necessary to retain the plate rigidly in any given 

 position. The plate, D, is provided with a slot, e e, and a milli- 

 metre-scale, S S, the dividing lines of which must, like those of 

 the tube to be calibrated, be very fine and truly equidistant. The 

 piece of gun-metal, E, which is provided with a vernier, carries 

 the reading-microscope, M, and can be moved along S S by 

 means of the rack and pinion p ; the movement is rendered 

 smooth and free from lateral displacement by the spring c, which 

 causes the ends of E to remain always in contact with the 

 straight edge of the slot. The thermometer-tube is fixed with 

 suitable screws under the path of the microscope, so that the 

 length of a thread of mercury can be easily measured by 

 placing the microscope so that its cross wire coincides first 

 with one end of the thread and then with the other, and noting 

 on the scale the distance between the two positions. 



The millimetres of the brass scale and those of the tube, if 

 marked off by different makers, will often differ a little in 

 length ; hence it is generally more satisfactory to obtain from 

 the glass scale the number of whole divisions occupied by the 

 thread, and to measure the terminal fractions only by the 

 microscope. 



Since the line on the outside of the tube is nearer the eye 

 than the thread of mercury inside the tube, it is clear that 

 when the microscope is adjusted to view the end of the thread, 

 and is then moved along until the cross wire coincides with 

 the nearest line, this last will be out of focus, and either the 

 whole microscope must be raised up or the distance between 

 the object-glass and eyepiece altered. Now, unless the in- 

 strument be constructed with great solidity, and much care 

 be taken to fit accurately all the moving parts, this adjustment 

 will probably alter the position of the optical axis, and so 

 render the measurements inaccurate. To avoid this diffi- 

 culty, I added a half-lens, L, fitted in the ordinary way on 

 a brass tube sliding on the end of the microscope. This 

 lens of course brings the focus of half the field nearer the 

 object-glass ; so that, by properly adjusting it, the divisions 

 are seen through the half-lens at the same time that the 

 mercury is observed through the unprotected part of the 

 object-glass. In this way all disturbance of the microscope 

 is avoided throughout the calibration, which is thus carried 

 out with much greater comfort and accuracy. 



