NATURAL PHILOSOPHY. 145 



specting the power of this instrument as follows : " Mr. Lyman de- 

 serves great credit for bringing so arduous an experiment to a success- 

 ful issue. His telescope, in point of optical power, is, so far as I know, 

 much in advance of anything heretofore achieved in this country. 

 Under favorable circumstances, it will separate double stars, distant 

 from each other by no more than ha/fa second of arc. To construct an 

 instrument that will accomplish this is no small matter. This fact is 

 worthy of being known, and I hope Mr. Lyman, bv the success of the 

 present effort, will find encouragement to aim at still greater success." 



ON A NEW MODE OF ILLUMINATING OPAQUE OBJECTS UNDER THE 

 HIGHEST POWERS OF THE MICROSCOPE. 



MR. C. BROOKE, at the British Association, described an arrange- 

 ment for best effecting the illumination of opaque objects under the 

 highest powers of the microscope. A parallel pencil of rays is obtained 

 by placing a camphene lamp (which, of all kinds of lamps, gives the 

 most intense illumination) in the principal focus of a combination of 

 two plano-convex lenses. This pencil is secured on the surface of a 

 small parabolic mirror, the vertex of which is truncated, so that the 

 focus of the mirror may be about 0.1 inch beyond the truncated edge. 

 The rays, which are converging to the focus, are received on the sur- 

 face of a small plane mirror which is attached to the bottom of the object- 

 glass, so that the surface of this mirror may be nearly level with the 

 lowest surface of the objectrglass. All the rays of light which subtend 

 any angle from that of the object-glass up to about 170 are thus ren- 

 dered available for the illumination of the object ; which, as it is illu- 

 minated by very oblique rays, must not be placed in a depression or 

 cavity of any kind. 



Mr. Brooke also described a new arrangement for facilitating the dis- 

 section of objects placed under the microscope. Two short pieces of tube, 

 one of them the size of the eye-piece, the other the same size as the 

 body of the microscope, are attached at an angle of about 4 to the 

 sides of a brass box containing a rectangular prism. The smaller tube 

 enters the body of the microscope, and the larger screws the eye-piece. 

 The image that enters the eye is now inverted in a plane passing 

 through the axis of the body and of the eye-piece ; and, in order to 

 erect the image, a cap is placed over the eye-piece, to which is attached 

 a small rectangular prism, having its axis in the plane in which the 

 image is already inverted. This arrangement provides a very convenient 

 position of the eye when the hands are engaged in manipulating an 

 object placed under the microscope. A rectangular prism has already 

 been introduced into the body of the microscope by Nachez ; but as 

 this was placed near the object-glass, it must, to a certain extent, in- 

 terfere with the definition of the objects. For the purpose of drawing, 

 a small piece of parallel glass is substituted for the rectangular prism, 

 placed in front of the eye-piece, through which the drawing-paper is 

 seen directly through two opposite surfaces, and the object is seen by 

 reflection from an outer surface placed at an angle of about 45 with 

 the axis of the eye-piece. The image inverted by the first reflection is 



