194 HOW TO SEE WITH THE MICROSCOPE. 



angle by the method previously given. Note again the 

 .angle obtained. Next, change the collar adjustment a 

 -division or two, and again measure the aperture. Com- 

 paring results, it will become obvious which of these 

 two positions of the collar corresponded to the larger 

 .angle. Should the latter measure prove the least, it 

 will be necessaiy to reverse the movement of the col- 

 lar, placing it a division or two from the previous posi- 

 tion but in the reverse direction, and by a few measures 

 of this kind, which, by the way, are quickly accom- 

 plished, the point in the collar adjustment correspond- 

 ing to the maximum aperture on the glass will be ascer- 

 tained with considerable precision. In the method just 

 described the primary object was to get an approximate 

 idea as to the point of largest aperture, and with the 

 least outlay of time, and subsequently, by actual trial, 

 to arrive at a more precise determination. The whole 

 process involves but little outlay of time, ten minutes 

 being quite sufficient for the purpose. 



With objectives of high balsam angles it will be neces- 

 sary to employ the genuine Wenham " reflex " illumina- 

 tor (angle of facet 26). With this instrument proceed 

 as has already been advised until, by the lateral move- 

 ment of the lamp at either the extremes, right or left, 

 the illumination commences to die away, the field being 

 blue or red, according to the position of the lamp. It 

 will generally be a saving of time to start with the 

 collar of the objective at "closed." Having found the 

 best position for the lamp, as we have before directed, 

 move it still a little further laterally until the field of 



