6 THE AMERICAN MONTHLY [Jannai-y, 



shows how many complete turns of the screw have been made in sepa- 

 rating the filaments, while the number to which the index points on the 

 graduated head of the screw shows what fraction of a revolution has been 

 made in addition. Besides determining, by means of the stage microm- 

 eter, the value of the divisions of the scale, it is necessary, for very 

 exact measurements, to make an accurate estimate of the thickness of 

 the filaments themselves, since, if this is not allowed for, a serious error 

 may be introduced, especially when the spaces measured are extremely 

 minute. Qiieckett, in his Practical Treatise on the Use of the Micro- 

 scope, has shown the theoretical possibility of measuring spaces as 

 small as the 1-800,000" with an ^" objective and this micrometer, but 

 there are practical difiiculties in the way of attaining this. 



The eye-piece micrometer can be introduced into the eye-piece when 

 making drawings by means of the camera lucida or any other of the 

 apparatuses vised for drawing with the microscope, and the divisions of 

 the micrometer sketched at the side of the drawing, as in working with 

 the stage micrometer. 



Abbe's form of apparatus, used for drawing, though more expensive 

 and complicated than the Wollaston or Beale, is a very convenient form 

 to work with. It consists of a cube of glass cut across diagonally. 

 The cut surface is silvered, all but a small patch in the middle, and the 

 two pieces are cemented together again. This is then fixed over the 

 eye-piece, in a suitable frame, so that the sheet of silvering inclines 

 downwards from left to right, with the unsilvered patch immediately 

 over the centre of the eye-piece. Attached to an arm extending out 

 towards the right is an ordinary silvered glass mirror, the plane of 

 which can be adjusted around a horizontal axis, parallel to the silvered 

 surface in the cube, and at the same vertical height as the centre of the 

 unsilvered patch. The drawing paper is placed under this mirror, 

 touching the foot of the microscope, and the mirror revolved, till, on 

 looking perpendicularly down the microscope at the object to be drawn, 

 through the unsilvered patch, the object and the reflection of that por- 

 tion of the paper next the microscope are seen together. The drawing 

 surface is reflected, first, from the mirror at the end of the arm on to 

 the cemented surface in the cube, the angle of which is so adjusted as 

 to throw the image vertically up into the eye. The point of the pencil 

 is very easy to see in this form of apparatus. With low objectives 

 and a good light, the field is lighter than the reflection of the drawing 

 surface. With stronger objectives it is necessary to reduce the bril- 

 liancy of the leflection of the drawing surface. This is done by the in- 

 sertion of one or two neutral-tinted glasses between the two mirrors, in 

 frames prepared for them. There is a third frame to hold a lens suited 

 to the eyes of any one with abnormal sight who uses the instrument, 

 the centre of the lens being opposite the unsilvered patch. This, of 

 course, has to be made to order. When the drawing surface is hori- 

 zontal the enlargement of the drawing is somewhat distorted, unless 

 the centre of the drawing is perpendicularly below the centre of the 

 mirror. 



If this cannot be arrived at by rotating the mirror on its axis, a longer 

 arm can be used to support the mirror, or, since the error is very slight, 

 the drawing surface can be inclined about 10° so as to bring it at right 

 angles to a line joining these two points. 



