INDICATOR FOR MICROSCOPES. 59 



microscope, the blank space being left for the portion of the 

 stage occupied by the aperture. 



The guide-lines upon the glass may be ruled with a fine- 

 pointed scratching diamond, and be rendered more visible by 

 having graphite or black lead rubbed into them. Lines ruled 

 in this manner will answer for all except very minute objects ; 

 but in consequence of the widening of the lines by the chip- 

 ping up of the glass due to unequal expansions and contrac- 

 tions, the lines often become too wide and irregular for very 

 accurate adjustments. In such cases admirable guide-lines 

 may be etched upon the glass with the vapour of fluohydric 

 acid, and can be made of any required degree of fineness. 

 The solution of the acid should not be employed for the 

 etching, as it gives lines which are too smooth and difficult 

 to see, and which will not retain the black lead if rubbed into 

 them. 



The power of the objective employed in determining the 

 position of an object for registration, should always be the 

 highest which can be conveniently employed ; while in search- 

 ing for an object already recorded, a power lower than that 

 employed in the registration may be used. The object then 

 must be in the field of view, and would be at the centre but 

 for slight errors in manipulation, or the want of perfect ad- 

 justment in the mountings of the object-glass. Care should 

 be taken to bring each object accurately to the centre of the 

 field of view, before recording it. It will then require an 

 error equal to half the diameter of the field of view to throw 

 it out of that field. For example, the field of view of my 

 ^-inch objective, made by Spencer, includes two divisions of 

 the Indicator, and hence an error of nearly one division might 

 be made in placing a slide upon the Indicator by means of 

 its recorded numbers, and yet the object would be found in 

 the field. 



It may happen that in transferring a slide from one Indicator 

 to another that the object when brought into sight by means 

 of its recorded co-ordinates will not appear well centred. If 

 this be due to slight differences in determining the centres of 

 the Indicator, and yet the record has been carefully made, it 

 is easy to correct for the difference in the following manner. 

 Move the Indicator ivith the slide placed at any recorded 

 position until the object comes into the centre of the field of 

 view, then secure the Indicator to the stage in this new posi- 

 tion, and all other objects recorded by the same Indicator 

 ought to be brought to the centre of the field of view by 

 means of the numbers as registered. 



The convenience of the Indicator for individual use may be 



