176 Transactions of the Society. 



From these it will be seen that some of the screws exceed in 

 diameter the maximum allowed in the specification ; others are 

 smaller than the minimum, the difference between the largest and 

 the smallest being 0*018 in., and that the shape of the thread has 

 greater differences than would seem possible. Exactly similar 

 differences occur in the nose-piece thread. 



In addition, the plain fitting above the thread of the object- 

 glass which should be -fa in. long is rarely considered ; the number 

 of threads or the length of screw is variable, yet both of these are 

 of vital importance in order that revolving nose-pieces may be made 

 with as short a fitting as possible. 



All the photographs have been taken with exactly the same 

 magnification and under the same conditions. 



The defects to which attention is called can be quite easily 

 controlled in any well ordered workshop, provided the tools are 

 accurate and definite correct standards are established. 



The present state of things is a reflection on the exactness of 

 the work of those who being connected with the production of 

 scientific instruments should make accuracy, in so important a 

 feature as interchangeability, a primary consideration. 



2. We will now consider the second point. Although it cannot 

 be denied that gross carelessness on the part of manufacturers is 

 accountable for such wide variations and departures from the stan- 

 dard sizes as have been shown in the photographs, I would submit 

 for consideration that the permissible latitude given by the Society 

 in their specification for the sizes of screw threads is far too great. 

 If you go to a workman and tell him that you want something made 

 accurately within 0*001 in. of a specified size he will probably ex- 

 press some doubt of his ability to do it, but if that same thing were 

 ordered to be of a definite size and no latitude were mentioned, 

 proper tools, gauges, etc., being provided, it would invariably be 

 found that the work had less than 0*001 in. of error. With skilful 

 workmen this has been found to be the case many times. Uncon- 

 sciously they are in the habit of working with very great accuracy. 

 Contracts are now regularly placed for large quantities of instru- 

 ments having interchangeable parts with a minus allowance of only 

 0-0011 in. 



The latitude given by the Eoyal Microscopical Society for the 

 top of the thread of the object-glass is * 003 in. — that is the differ- 

 ence between the maximum size 0*7982 in. and the minimum 

 0*7952 in. 



Similar latitude is given in the nose-piece size, namely * 803 in. 

 to 0*8 in. at the bottom of the thread. It is therefore possible, 

 taking the extreme limits of the object-glass thread and the corre- 

 sponding nose-piece thread, for a difference of * 0078 in. to exist. 



Various makers have apparently interpreted these limits in 

 different ways. Some have used less than the minimum size, 



