148 ANNUAL OF SCIENTIFIC DISCOVERY. 



" The construction of this microscope is based upon the principle of 

 placing the objective glasses beneath instead of above the objects to be 

 examined. The tube in which the eye-glass is, is thereby five degrees 

 from the perpendicular, and the ray of light undergoes a deflection of 

 one hundred and forty degrees before entering the eye. The deflection 

 is produced by a four-sided prism with the angles 55, 107^, 52^, 145, 

 the ray of light passing through the objective glass down ; into the 

 upper side of the prism it penetrates and is subjected to two total reflec- 

 tions, and passes out of the fourth surface upwards, making an angle of 

 thirty-five degrees with the perpendicular. The eye regards, almost at 

 the same moment, the object itself, on the stage of the microscope, and 

 the image of the same in the instrument ; and here the advantage of 

 the instrument is seen for chemical purposes, for which it was origi- 

 nally intended. It is, however, not confined to this, for, owing to the 

 convenience it affords for arranging the illumination, it is well adapted 

 to much general use. The new plan of measurement is to introduce a 

 micrometer into the tube of the microscope at any stage of the obser- 

 vations, by means of an arrangement placed so that the micrometer 

 comes within the plane of the foci of all the eye-pieces used in the 

 instrument. The method of measuring the angles of crystals is by 

 having a graduated circle in the outer part of the tube of the micro- 

 scope, and passing from the tube carrying the eye-pieces, which has a 

 circular movement independent of that of the graduated circle. The 

 manner of measuring the angles is a3 follows : Introduce the mi- 

 crometer, turn the eye-piece until the lines on the micrometer are 

 parallel to one side of the angle to be measured ; then, leaving the eye- 

 piece, turn the graduated circle until the index on the eye-piece is at 

 zero ; this done, turn the eye-piece until the lines on the micrometer 

 are parallel with the other side of the angle to be measured, and, in 

 regarding the circle, the degrees of the angle passed through will be 

 seen. The method is convenient, and more precise than any known." 



EFFECT OF THE HEAT OF THE SUN UPON THE PERPENDICULARITY OF 



BUNKER HILL MONUMENT. 



IN the experiment of swinging a pendulum in the shaft of Bunker 

 Hill Monument, for the purpose of illustrating the experiment of Fou- 

 cault, it was observed that the ball of the pendulum, when at rest, was 

 not always over the same point in the floor. The careful consideration 

 of all the conditions of this fact resulted in ascribing it to the unequal 

 expansion of the sides of the monument, in consequence of unequal 

 exposure to the sun. Upon observing carefully, it was found during 

 clear days that the motion of the ball in the morning was to the west- 

 ward, at noon to the north-west, and at evening to the east. It was 

 further observed that on days when the sun was obscured by clouds, 

 no motion of the ball on its index-point occurred. It was still fur- 

 ther observed on one occasion, during a sudden shower, accompanied 

 with strong wind from the south-east, to move in the space of a very 

 few minutes a quarter of an inch to the eastward. Observations were 

 recorded through several weeks, and no doubt remains that a cause 



