TEST OBJECTS. 46^ 



inch thick, ami of sufficient radius to suit the length of an 

 ordinary compound body, the curved edge is graduated into 

 180°; upon the flat surface of the semicircle a strip of wood or 

 index, an inch or more in breadth, is made to turn upon a pin, 

 as seen in the figure ; on the upper surface of this index two 

 crutches are fastened to receive a compound body, provided 

 with an eye-piece of the usual Huyghenian construction ; the 

 object-glass, whose angle of aperture is about to be measured, 

 is screwed to the opposite end of the body to that of the eye- 

 piece, as in the ordinary compound microscopes. The method 

 of using this instrument is the same as that of the microscope 

 described by Mr. Lister; a candle is placed a few yards off, 

 and the instrument is so arranged that when the index points 

 to zero the field of view should be vertically bisected ; if now 

 the index be turned so far that the opposite half of the field is 

 illuminated, the number of degrees passed over wUl give the 

 measurement of the angle of aperture of the object-glass, 

 required. 



In the early days of achromatic combinations, the angle of 

 aperture was small, and it is very interesting to observe how 

 steadily our first-rate opticians have been progressing towards 

 the utmost hmit of conceivable perfection. As the subject is 

 so important, the author has thought proper to introduce in 

 this place an account of the progress Mr. Ross has made in 

 transmitting angular pencils in object-glasses of different foci 

 since the year 1832, for which valuable information he is 

 indebted to Mr. Ross himself, who has kept an accurate 

 account of the same. "In the year 1832 he made for R. H. 

 Solly, Esq., an object-glass, consisting of two double achro- 

 matic coinbinations, which was of an inch focus, and transmitted 

 a pencil of 14°. In 1833 he constructed triples after the plan 

 of Tulley, having an angular aperture of 18°. In 1834 he 

 made an object-glass of one-fourth of an inch focus, which 

 transmitted an angular pencil of 55°; this glass is now in the ' 

 author's possession. In the beginning of the year 1836 he 

 constructed a triple inch glass, with an angular aperture of 

 15°, with cemented surfaces ; and towards the end of the same 

 year he made glasses of one-eighth and one-tenth of an inch 

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