36 JOURNAL OF THE [February, 



tween the stage and the mirror, its image will be exhibited by 

 every lens. Also if a small letter, figure, or picture, in black, 

 with a clear, white background, be placed one or two inches be- 

 low the stage, and a strong light be condensed upon it, it will be 

 seen with tolerable distinctness. Such objects are, however, 

 much more sharply defined if first cut out, and then pasted upon 

 a thin cover-glass, which may be mounted on the substage. In 

 this situation the object is illumined by light reflected from the 

 mirror. The effect will be still better if a slip of ground glass 

 be interposed between the object and the mirror, so as to shut 

 off the image of the lamp, if lamplight be employed, or of dis- 

 tant objects, if daylight be used. The eye of a mosquito will 

 show two or three hundred pictures of a person, in silhouette, 

 with great distinctness, provided you have a window so situated 

 as to allow light from the sky or from a white cloud to pass un- 

 obstructed to the mirror. The person must stand at a distance 

 of five or six feet from the microscope, and with the profile of 

 his face in clear relief against the sky. The plane mirror must, 

 of course, be used. 



I have recently been much interested in examining the struc- 

 ture of the eye ol Limulus, or the Horse-shoe Crab, which, though 

 compound, is quite different, in some particulars, from that of 

 insects. The exterior of this eye is perfectly smooth, and con- 

 sists of a transparent horny coat of considerable thickness. 

 The concave interior surface is studded with lenses varying in 

 form from plano-convex, near the centre, to conical and parabo- 

 loid, toward and at the periphery. These lenses are so placed 

 that their optical axes converge to a common point situated in a 

 plane a little below the base of the whole eye. This point, with- 

 out doubt, is occupied by the retina, or the extremity of the op- 

 tic nerve. Good multiple images will be made by this eye if a 

 small disk cut from the central part be used, the eye being flat- 

 test at that point and the lenses least conical. From any other 

 part of the eye it would be extremely difficult to cut a disk that 

 would not, in consequence of the oblique position of the lenses, 

 greatly distort the images. 



Multiple images may be formed under the microscope in many 

 other ways than by the use of compound eyes. The minute 

 plano-convex bodies of water produced by breathing on a slide, 

 will display good images of any small object supported above 



