32 
GiBBONs_, on a New Method of Micrometry. 
micrometers. 3. Scales of specific divisions applied to the 
object and viewed with it. Of this class are the ordinary 
glass stage micrometers^ which, however, will only do for 
powers that are low enough to admit of the scale and object 
being nearly in focus at the same time ; except, indeed, for 
very rough measurements, when the object is first viewed, 
and the space it occupies retained in the mind's eye while 
the scale is rapidly brought into focus. 4. The projection of 
the magnified image- on a sheet of paper by means of the 
camera lucida, and 1 he measurement of it by lines previously 
drawn upon the paper, or by the application of a determinate 
scale. All these methods require much previous calculation, 
and most of them need special adjustment and peculiar 
delicacy in the scales themselves, besides limiting the ob- 
server to the particular lenticular combinations for which they 
were contrived. 
My plan, which may be added as a fifth class to the above 
category, consists in the application of a previously magnified 
scale to the object itself, or rather to its magnified image. 
I had long been in the habit of using a common drawing 
scale or foot rule for the estimation of magnifying powers. 
I laid the rule on the stage, and viewed one of its divisions 
through the instrument, and the whole rule with the dis- 
engaged eye. The distance to which the magnified division 
extended on the normal scale was then a matter of very 
simple calculation, and was registered for each power. The 
same method served also to measure the field. When I 
wished to measure an object, I observed it in the usual way, 
against the same rule viewed with the disengaged eye, and 
the proportion, fractionally rendered by the previously 
registered formula, gave the measurement. It then occurred 
to me (nearly two years since) that the process might be 
further simplified. 
The following extract from my note-book shows the method 
I employed, and the steps by which I arrived at my present 
plan : 
" Lay on the stage of the microscope, beside the object, a 
scale of any kind sufiiciently long to include the diameter 
of the field (a foot rule with tenths serves very well) . View 
the object against this by the use of both eyes, one at the 
instrument and the other outside, the latter directed toward 
the rule : a little practice will render this easy. Note the 
measurement. Then substitute a stage micrometer for the 
object, and observe in like manner, so as to determine the 
value of its divisions relatively with those of the rule. (This, 
of course^ need only be done once for each combination.) 
