The Microscopic Compass. 239 



sights may be considered as a part of the cylinder of which the box 

 is made. The sight K S which is to be used next to the eye, is made 

 precisely as in the common compass, having several holes placed ver- 

 tically, and these connected by a slit one-thirtieth of an inch wide. 

 (Fig. 3.) The opposite sight V J is slitted and perforated in a simi- 

 lar manner from V to Q chiefly for the purpose of reversing for ad- 

 justment. Its main use however is independent of these slits, it is in- 

 tended to support at its top an inclinable reflector (M), the inclina- 

 tion being directly towards the first sight K S ; and immediately be- 

 low the reflector, a single lens or microscope (L), placed horizontal- 

 ly one inch above the card and having an inch focus. In these two 

 parts, the lens and the reflector, lies the peculiarity of the instru- 

 ment. 



Every philosophical reader is now enabled to understand the use 

 of the instrument. The rays of light passing divergingly upward 

 from any point of the card to the lens, pass through it and emerge 

 parallel above it, where meeting with the reflector (M) inclined to an 

 angle of forty-five degrees, they are reflected horizontally through the 

 opposite sight at (O). The eye at the sight (O) would of course see 

 the card in the direction in which the rays were last received ; that is, 

 in a horizontal direction. As the rays would be rendered parallel, the 

 card would appear at a great distance and magnified. 



The card thus reflected would appear in a vertical position, in 

 the position of a full moon just risen, with that point which is really 

 farthest from the eye as the lowest point in the reflected image. It 

 should be remarked that the whole of the card is not seen at once in the 

 reflector, but only about ten degrees of the lowest part of it. (Fig. 4.) 

 The silvering of the reflector being removed from the lower half, (from 

 M towards J) any distant object can be seen through the opening 

 while the card is reflected from the upper part, and by inclining the 

 reflector on the hinge (J) the graduated edge of the reflected card 

 can always be brought to visual contact with the object whose bear- 

 ing is at once read, as it were, on itself. The rays of light coming 

 from a distant point are nearly parallel, and those coming from the 

 card are made so by passing through the convex lens ; the conse- 

 quence is that both the degrees on the card, and the object are seen 

 perfectly clear as if equally distant. This would not be the case if 

 the card were reflected without the interposition of the lens. The 

 degrees on the card and the object would then be in the condition of 

 one object within three inches of the eye and another at an indefinite- 



