MEASURING THE MAGNITUDE OF MICROSCOPIC OBJECTS 209 



On Defining the Position and Measuring the Magnitude of 

 Microscopic Objects. By the Rev. W. Hodgson, M.A., 

 Incumbent of Brathay. 



It is a matter of great practical convenience that the Micro- 

 scopical Society have adopted, as one of their standards for 

 size, a slide which measures exactly three inches by one. Of 

 the three square inches thus fixed upon, the middle square 

 inch is that which alone is employed to carry the object. 

 The inch on the right hand is appropriated to the label, or 

 name ; that on the left may be given up to registering the 

 position of any object, or may be left unoccupied altogether. 

 The middle square inch, therefore, is the only one to Avhich 

 any measurements need refer. Let, then, the bounding lines 

 of this square inch at the bottom and on the left hand be 

 taken as what geometers would call the axes of rectangular 

 co-ordinates, or what, in the language of map-makers and 

 geographers, would be the equator and the first meridian, and 

 let the measurements be made in hundredths of an inch. 



If an object, P (fig. 1), upon a slide represented by the dark 

 thick lines of the figure, were distant from A C by 67-lOOths 

 of an inch, and from A B 39-lOOths, a geometer would at 

 once understand its position from the values x = 67 and 

 y = • 39, and the geographer would know what situation was 

 intended by long. 67° and lat. 39°. 



In order, therefore, to define the place of any object on a 

 slide, two numbers are all that are essential. 



The next step is to bring the point thus defined into the 

 centre of the field of the microscope. In all modern micro- 

 scopes, of even the most moderate pretensions, the optical 

 axis of the instrument will, if produced, pass through the 

 centre of the stage ; so that there exists already, in every such 

 microscope, a fixed point for the origin of co-ordinates, or 

 for the intersection of our microscopical equator and first 

 meridian. With the assistance of a diametral cobweb-line in 

 the eye-piece of the microscope, the point A (fig. 1) of the 

 slide, which is situated exactly one inch from its left-hand 

 extremity, may he brought accurately to the centre of the 

 stage, so that A B and C D A coincide respectively with the 

 horizontal and vertical lines through that point. Tlie mea- 

 surements, therefore, which before were referred only to the 

 middle square inch of the slide, may now, by means of gra- 

 duation, be transferred to the stage of the microscope, or to a 

 supplementary stage with the name of " a finder." 



In the simplest case of tl plain stage, a piece of sheet brass, 



