I 



OF ARTS AND SCIENCES. 355 



The screw in use is slightly oiled with an unguent consisting of 

 equal parts of beeswax and tallow, with about -j^ part of clock oil 

 added. 



To facilitate exact setting with the screw, a smoothly turned and 

 thin wooden disc 8.5 cm. in diameter slips over the screw head, to be 

 clasped at its opposite edges by the fingers and thumb, iu turning 

 the screw. The whole micrometer screws to a stage plate, which may 

 be readily slipped into the grooves cut in the stage of the microscope 

 stand ordinarily to receive the object-holders. 



The results given of the measures of short standards by this appa- 

 ratus would be of little interest, unless accompanied by the results of 

 an investigation of the errors to which a single setting of the screw 

 is liable. 



A simple method of investigating at once the errors depending upon 

 the graduation of the head of the screw, of the variation in different 

 parts of the same revolution, and of any cumulative error in the 

 length of one revolution at different distances from the assumed zero 

 of the scale, is to use a single band in the manner described below of 

 the width of the value of one revolution, consisting of as many lines 

 ruled on glass as there are units in the denominator of the fraction 

 expressing the value of the smallest fractional part of the head to be 

 considered. 



The first line of this band, when the whole band has been passed 

 over, is brought successively back to the index in the eye-piece, which 

 should be perhaps two parallel lines nearly the same distance apart as 

 the apparent width of the line on the stage micrometer as seen in the 

 field of view. One of the screws of the mechanical stage is used for this 

 purpose. This band should have lines enough upon it to have two con- 

 secutive ones in the field at once with a high power objective, in order 

 to have its errors investigated with an eye-piece micrometer, and inde- 

 pendently of the screw. It should be borne in mind that in this case the 

 measures should be made in the same part of the field, to avoid errors 

 arising from the unequal distortion of the eye-piece lenses. We thus 

 avoid the otherwise necessary examination of a long scale of lines ; 

 and it is my opinion that it is safer to make the more numerous settings 

 required by this method, than to trust to any inexhaustive treatment 

 of a series of many lines, such as would be necessary without a consid- 

 erable expenditure of time. 



In determining the mean value of one revolution, we shall derive 

 an advantage in using the mean of the ten settings for terminal 

 points. 



