MEMORANDA. 



207 



two lenses of the eye-piece. It is formed with two penta- 

 gonal faces, a, b, c, d, e, and a, b, h, g, f, joined at a 

 common edge, a, b, and connected at the opposite end by 

 two facets, c, d, g, h, and d, e, f, g. The solid formed 

 by these surfaces represents pretty nearly a double wedge, 

 of which the edges, a, b, and d, g, are perpendicular to each 

 other. The way in which the erection of the image is ef- 

 fected may be readily conceived. The rays emerging from 

 the field-glass enter by the lower surface, and are reflected at i 

 upon the face a, b, h, g, f, from which they are again re- 

 flected upon the lower surface at the point k, and thence to the 

 point l upon the vertical face c, d, g, h, and lastly at the 

 point m upon the other vertical face, d, e, f, g, from which 

 the image, normally and completely erected, is again sent 

 back to issue by the superior surface upon which the eye- 

 glass is placed. Consequently the 

 two surfaces, looking towards the 

 lenses of the eye-piece, are at the 

 same time surfaces of reflexion and 

 of transmission. All the reflexions 

 are total, except the first at i. 

 The upper surface is coated over 

 the space between a, b, h, f, and 

 left free between f, g, h, to allow the 

 image to pass. 



The loss of light is inconsiderable, 

 owing to the energy of the con- 

 vergent pencil put in action in this prism, which may 

 be applied to magnifying powers far greater than those usu- 

 ally employed in the minutest dissections. The combination 

 represents a weak ocular ; so that when the eye-piece is ap- 

 proached to the objective, as small a magnifying power as 

 may be wished is obtained. — Nachet. 



On a Dark-Ground Illuminator. — I have found, that with the 

 little instrument here represented, 

 effects may be obtained as remarkable 

 as those produced by the ingenious 

 paraboloid of Mr. Wenham, which in 

 its construction demands so much care, 

 and is too large to be applied to certain 

 instruments. 



Our apparatus consists of a simple 

 cone of glass, whose summit is di- 

 rected towards the mirror, and base towards the object. 

 This base forms a segment of a sphere, and the centre 



