26S 



HA RD WICKE'S SCIENCE- G OSSIP. 



My own working angles of slope have been 40 

 and 30 . From the above formula it was easy to 

 compute by logarithms the values of /', which are 

 given in the following Table. 



Table showing the Relation between (1} the Angle formed by a 

 ray with the Axis of the Mirror; (2I the Angle formed by 

 that ray with the Axis of the Microscope ; (3) the Horizontal 

 Angle on the Tray [the mirror being in the axis of the Micro- 

 scope). 



Angle with 

 Axis of 

 Mirror 



With Axis of 

 Microscope 



Horiz o n t a I 

 Angle when 

 slope is 40° 



When 30 



:h| 

 °. f , 



25" 



S°" 



3° 



6o° 



40" 



8o° 



50" 



6o° 



90" 



J32°57', 



49°'5', 76°54', 9°°, 103 



7° 



140 



1S0 



4 2° 5 ', 54°44', 78°26', 90°, ioi j 4 ', i2 5 °i6', i S 2°i2' 

 By the supposition, we are proposing to throw our 

 parallel pencil on the mirror at an angle of 30 . If the 

 tube slopes at an angle of 40 with the horizontal, a 

 line must be drawn on the tray representing a hori- 

 zontal angle of 49 15', and the lamp-flame be placed 

 over that line. If the slope be 30 , another line 

 must be drawn at an angle 54 44', and the lamp be 

 placed thereon. 



The reader will now understand the lines marked 

 upon the tray in Fig. 166. The continuous lines are 

 for use when the tube is sloped at an angle of 40 ; 

 the dotted lines are for a slope of 30 only. He 

 will observe that the difference of position in the 

 different pairs of lines diminishes as the lamp ap- 

 proaches the rectangular position "for 45°," and 

 there vanishes altogether ; an important reason for 

 preferring that position in our ordinary work. He 

 will also notice that after passing "for 45 ," the 

 difference re-appears, but that the positions of the 

 two lines are reversed. An examination of the Table 

 will explain this change of position. 



In lining out the tray, a black spot (c, Fig. 166) 

 is to be first marked in the position most convenient 

 for bringing the centre of the mirror over it when we 

 slide the instrument on its platform. From it, lines 

 are to be drawn, forming with a line A c, beneath the 

 tube of the microscope, such horizontal angles as are 

 given in the Table, or as may be calculated from the 

 formula for other angles of slope than those to which 

 the Table relates. Whenever the slope of the tube is 

 altered, or the mirror is raised or lowered, the centre 

 of the mirror must be again brought over the spot c, 

 and the lamp must be altered both in height and 

 position. 



We have now settled the focal distance of the 

 mirror, and the angular position and height of the 

 lamp, but have not determined at what distance from 

 the mirror the lamp should be placed. This can be 

 more conveniently discussed in connection with our 

 next subject — the correct position of the bull's-eye. 

 ( To be continued. ) 



We regret to have to record the death of Dr. F. 

 Guthrie, Professor of Physics at the Royal School 

 of Mines, at the comparatively early age of 53. 



A WASP'S NEST. 



HANGING outside my dining-room window, 

 protected from wind and rain by the project- 

 ing woodwork, is a wasp's nest. Do not be alarmed, 

 it is no headquarters of a busy crowd of irritable wasps, 

 but the dainty home of a queen wasp, who, having 

 survived the winter, set to work on the 3rd of June 

 to found a new colony. In size, shape, and colour, 

 it resembles a large walnut suspended by one end to 



U\\\ 



Fig. 168.— Wasp's Nest. Full size. 



Fig. 169. — Section of Interior of Wasp's Nest. 



the woodwork above the window, with an entrance 

 at the other end. It is composed of comb, not made 

 of wax like the bee's, but of very thin coarse paper, 

 manufactured by the queen from decayed wood. She 

 first suspended to the window frame by a firm stem 

 the commencement of five cells, then she built round 

 them, springing from the stem, a shell of the comb, 

 leaving a small hole at the lower end. As soon as 

 this was finished she again commenced from the stem 

 a second shell, covering it almost, but not quite touch- 

 ing the first, leaving the entrance as before. 



A third and half a fourth shell were added in the 



