December i6th, rpo2.] PROCEEDINGS. xiii 



elaborate for the elementary student, who is probably not familiar 

 with the principle or use of a compound microscope. 



In the simple apparatus to be described, no new principles 

 of measurement are introduced, nor is any attempt made to 

 attain the accuracy of the better instruments ; but the parts are 

 all clearly visible and easy to understand, readings can be taken 

 by verniers to one-hundredth part of a centimetre or inch, and 

 two methods of avoiding parallax errors are employed. 



The apparatus consists of a wooden base, on the upper sur- 

 face of which are fixed horizontally two parallel scales half a metre 

 long, divided in tenths of inches and centimetres respectively 

 (Fig. i). Between the two scales the base has a longitudinal 



Fi£. I. Plan (leus removed), 

 V-shaped groove, in which the tube to be calibrated rests, the 

 sides of the groove being made white for illuminating purposes 

 {Fig. 2). A circular hole is cut through the base at one part of 



Fig. 2. Section at AB. 



the groove, and a spring clamp is arranged for holding in this hole 



