46^ K. M. NELSON ON DETERMINATION OF FOCt. 



lens, the magnifying power becomes less, and _ becomes greater ; 



m 



when the magnifying power becomes 1 — that is, when the image 



is the same size as the object — then ± =f, and p =2/, or twice 



m 



the focus. The value of j), in terms of 5, may be found in the 



same manner as we obtained that of d above. Thus, s = d + p, 



d =2^ wi; combining, we have 



Exception may be taken that the term " lantern lens " has been 

 more often used than that of " microscope objective." The reason 

 for this is that the same argument and language is applicable to 

 both, and lantern examples are, perhaps, more suitable for expla- 

 nation. Further, this objection should not be made by the 

 members of this Club, as they now possess a lantern of their own. 

 It is, therefore, all the more incumbent on them to understand the 

 optical theories connected with it as well as with the microscope. 



Finally, let me point out that formula (v.) contains all the 

 necessary data for determining the distance of either the image or 

 the object for purposes of enlargement or reduction. The rule is : 

 Add one to the times of enlargement, and multiply the result 

 by the equivalent focus. The back focus, p, has been fully dis- 

 cussed above. 



In putting forward these few elementary notes, I trust that they 

 will prove equally useful to all three branches of workers, viz , 

 microscopists, lanternists, and photographers. 



