22 Transactions of the Royal Microscopical Society, 
the field of vision of the objective oj, the possible difference of 
centering being eliminated by taking the mean of the reading of 
both indices. 
It is important in this method of measuring that, by the 
arrangement described, all “ false rays,” f e. all rays which do not 
act in the formation of the ordinary microscopic image, which the 
objective x would produce, are excluded. If the observation is 
made with the naked eye, the pupil of the latter (which by its 
central position at the end of the tube will coincide with the ordi- 
nary image as seen in the eye-piece) acts as a diaphragm for 
this purpose. In the case of the auxiliary microscope the same 
effect is produced by a diaphragm above the achromatic lens, be- 
longing to the apparatus. In observing the indices on the disk by 
the auxiliary microscope, this diaphragm excludes all rays besides 
those which would form the ordinary microscope image in the 
middle part of the field of vision. Therefore this diaphragm forms 
an essential part of the apparatus, and must be specially adjusted 
to the objective B, in diameter and position, in order to fulfil its 
task. 
In the two scales the position of every line has been calculated, 
the calculation being based on the measured index of the crown 
glass forming the disk. 
