140 



of the focal fine adjustment should be placed about midway between 

 the two extremes. 



The only novelty in this communication is the pushing-screw, 

 which acts as a fine adjustment in bringing a line on the micrometer 

 to coincide with the edge of the object to be measured. This I ex- 

 hibited to the Society at a former meeting, and the president after- 

 wards suggested that I should prepare a written description of it, that 

 it might, if the council thought fit, be published in the 'Transactions.' 



I have found so many working microscopists without any clear 

 ideas on the subject, that I have gone much further into it than would 

 have been necessary merely to describe this little contrivance : and I 

 have done it in the hope that, although the details will to many ap- 

 pear tedious, there are some few to whom they may be of service. 



