GENEEAL PKELIMINAKY PKEPAKATIONS 197 



millimetre scale. The reason why it is advisable to take a 

 long camera-extension is that the distance should be reckoned 

 from the posterior principal focus of the lens, and this is not 

 a point easy to determine. Over a long distance the error 

 becomes so small that it is not really of great moment, and the 

 distance may be measured from the front lens of the objective. 



Another small error is introduced owing to the fact that 

 the farther the image is from the objective, the nearer the lens 

 has to be focussed to the object to get the projected image 

 into correct focus, but this again is not sufficient to disturb 

 the result for ordinary work. It must be carefully noted in 

 making any estimation of magnification that in cases where an 

 ocular is used it only holds good for the particular tube-length 

 with which the determination is made. It is not advisable 

 in determining magnification by the aid of a micrometer to 

 rely upon the distance between any two rulings, but rather 

 to take a number and then to take the mean of these ; there is 

 usually some error in individual rulings, and this is reduced to 

 a minimum if the mean is taken of several. Where ordinary 

 oculars are used the determination must be made with each 

 objective in combination with each ocular. 



As examples of magnification to be obtained with varying 

 types and makes of objectives, the following are the results 

 with some in the hands of the writer. Table IV is really an 

 abbreviation of one kept at hand for reference when any given 

 magnification is required. 



TABLE IV 



