194 president's address. 



experiment on this still dimly understood phenomenon ? The 

 last in the list is the most bizarre piece of apparatus that has 

 appeared in the microscopical world for many years. Of course, 

 in the annals of a patent office, any number of the most mad 

 and impossible inventions may be seen, but it is almost in- 

 credible that such a device as that of Heir A. van Delden 

 should appear seriously in an}' scientific journal. Strange as it 

 may seem, it has nevertheless been published in Germany, and 

 is extracted without comment and inserted with three figures in 

 the "Journal R.M.S."* The title of the article is " An Auxiliary 

 Apparatus for the Adjustment with Immersion Objectives." 

 The appliance, in spite of its high-flown title, is of the nature 

 of a safety stop to prevent the owner of the instrument either 

 crashing through the slip or driving his front lens through a 

 back combination while focussing an oil immersion objective. 

 The following is a description of the manner in which this is 

 effected : —A block is fastened to the body of the microscope 

 above and in a line with the top of the limb. A 50-thread 

 micrometer screw passes through this block in a vertical direc- 

 tion. The head of the screw is divided, and alongside there is 

 a graduated post, on which can be read whole revolutions of the 

 screw head. When the body is racked clown by the coarse 

 adjustment this micrometer screw comes in contact with the 

 top of the limb and prevents the body being racked down any 

 further. If it is required to lower the body still more the 

 micrometer screw must be raised by unscrewing it, when of 

 course the body can be racked down until the screw is again 

 brought into contact with the top of the limb. Before the 

 apparatus can be employed it is necessary to find the co-efficient 

 of the instrument; this is arrived at in the following manner. 

 A particular low-angled dry objective is placed on the nose- 

 piece, and an object on the stage is brought into focus. The 

 micrometer screw is now lowered until its point butts against the 

 top of the limb, the number of revolutions and parts of a revolu- 

 tion of the micrometer screw are read off on the post and screw 

 head respectively and noted, after which the fine adjustment of 

 the microscope must not be touched. The dry objective is then 

 removed and the oil immersion put in its place ; this is now 



* Tom. cit., p. 585, three figs. 



