210 HODGSON, ON MEASURING THE 



zinc, card, or other material, four inches by two, is prepared 

 with a central hole of one inch in diameter, and fitted, with 

 ledges, pins, springs, or some such contrivance, to the stage 

 of the microscope, in such a condition as to allow the glass 

 slide to be moved over it in various directions without dis- 

 turbing its position. An empty slide, on which the line A C 

 has been drawn with a common writing diamond, is brought, 

 with the help of the cobweb line in the eye-piece, into the 

 situation shown at fig. 1. The lines H I, H K, which are 

 merely the prolongations of the lines A H, G H given by the 

 edges of the slide, are graduated into tenths and hundredths 

 of an inch, and numbered both ways from H. 



In order to bring an object, P, defined by long. 67° and 

 lat. 39°, into the centre of the field, place the slide so that 

 the edge G H cuts the scale H I at the 67th division, 

 while the edge H A cuts the other scale at the 39th division, 

 as shown in fig. 2. It is desirable to have the graduation of 

 H K repeated at L M, as an assistance towards keeping the 

 line A B parallel to its former position. With a plain stage, 

 the above plan is sufficient for those whose eyes can deal 

 readily with hundredths of an inch, and whose fingers can 

 easily make adjustments with the requisite nicety. 



When the stage is fitted (as in the Students' Microscopes of 

 Messrs. Smith and Beck) with dove-tailed grooves, in which 

 a frame for steadying tlie slide moves up and down, the 

 position represented in fig. 1 is obtained with more ease than 

 with a stage entirely plain. There is also a further advan- 

 tage in this case, by which minute dividing may be dispensed 

 with, without any sacrifice of accuracy. The moveable frame 

 may have attached to it a piece of thin brass, about an inch 

 broad, and on this the graduation H I, fig. 1, may be replaced 

 by a diagonal scale reading to hundredths of an inch, while 

 tlie edge on the right hand, or in some other convenient posi- 

 tion, may carry a vernier, divided as in the common baro- 

 meter, by which divisions of tenths on the edge of the stage 

 may be read to hundredths, instead of having recourse to such 

 minuteness as is required for the plain stage at H K. 



Indeed, minuteness of division may be altogether dispensed 

 with, even for the plain stage, by adopting the form of Indi- 

 cator represented in fig. 3. The principle involved is precisely 

 the same as that employed in fig. 1, and the only difference in 

 the application of it consists in substituting two diagonal 

 scales reading to hundredths of an inch, for the other smaller 

 and less convenient graduations. The divisions in this case 

 are so large that, with a flat rule and a writing diamond, the 

 lines may be readily drawn in a few minutes upon a piece of 



