NATURAL PHILOSOPHY. 153 



have a glance, as it were, into another visible world, in which external 

 objects and our internal perceptions have no longer their habitual 

 relations with each other. Among the remarkable illusions it occasions, 

 the following are mentioned : The inside of a tea-cup appears a solid 

 convex body ; the effect is more striking if there are painted figures 

 within the cup. A china vase, ornamented with colored flowers in 

 relief, appears to be a vertical section of the interior of the vase, with 

 painted hollow impressions of the flowers. A small terrestrial globe 

 appears a concave hemisphere ; when the globe is turned on its axis, 

 the appearance and disappearance of different portions of the map on 

 its concave surface has a very singular effect. A bust regarded in 

 front becomes a deep hollow mask ; when regarded en profile, the 

 appearance is equally striking. A framed picture hung against a 

 wall appears as if imbedded in a cavity made in the wall. An object 

 placed before the wall of a room, appears behind the wall, and as if an 

 aperture of the proper dimensions had been made to allow it to be 

 seen ; if the object be illuminated by a candle, its shadow appears as 

 far before the object as it actually is behind it. 



BINOCULAR MICROSCOPE. 



THE following notice of a new form of microscope, is taken from 

 the New Orleans Medical Register. " At a meeting of the Medical 

 Society, Oct. 1852, Prof. J. L. Riddell called the "attention of the 

 society to an instrument of his own invention and manufacture, which 

 promises to be of incalculable advantage in microscopic researches, 

 especially in the prosecution of microscopic anatomy and physiology. 



" He remarked that he last year contrived, and had lately constructed 

 and used, a combination of glass prisms, to render both eyes serviceable 

 in microscopic observation. The plan is essentially as follows : Behind 

 the objective, and as near thereto as practicable, the light is equally 

 divided, and bent at right angles and made to travel in opposite direc- 

 tions, by means of two rectangular prisms, which are in contact by 

 their edges, that are somewhat ground away. The reflected rays are 

 received at a proper distance for binocular vision upon two other 

 rectangular prisms, and again bent at right angles, being thus either 

 completely inverted, for an inverted microscope, or restored to their 

 original direction. These outer prisms may be cemented to the inner, 

 by means of Canada balsam ; or left free to admit of adjustment to suit 

 different observers. Prisms of other form, with due arrangement, may 

 be substituted. 



" This method proves, according to Prof. Riddell's testimony, equally 

 applicable to every grade of good lenses, from Spencer's best sixteenth, 

 to a common three-inch magnifier, with or without oculars or erecting 



o *~^ 



eye-pieces, and with a great enhancement of penetrating and defining 

 power. It gives the observer perfectly correct views, in length, 

 breadth and depth, whatever power he may employ ; objects are seen 

 holding their true relative positions, and wearing their real shapes. 

 In looking at solid bodies, however, depressions sometimes appear as 



