Mercury Globules as Test Objects. By J. W. Gordon. 13 



It will now be obvious that every reflecting surface in the 

 instrument must send a certain amount of light back to the stage. 

 It is not, however, every such surface which concentrates the light 

 sufficiently upon the globule to produce a visible image there. It 

 is a selection only of the reflecting surfaces which thus produce 

 images such as can be examined in the Microscope. I imagine, 

 however, that every separate lens must have at least one surface 

 which thus yields a visible image. That, however, is too com- 

 plicated a problem for me to be able to discuss it to advantage. 

 What is quite clear from a mere inspection of the images so 

 formed is that almost, if not entirely, all the lenses contribute to 

 the collection of images. The multiplicity of such images and 

 their disposition close behind one another — when a very small 

 globule is used as the reflecting mirror — are, indeed, the principal 

 defects of this system of examination. The images will, many of 

 them, be found to come into view simultaneously, and then if, as 

 often happens, they overlap but do not coincide with one another, 

 a confused image results in which it is not easy to discern the 

 outline of the object globule. In the case, however, of a well- 

 constructed lens, the light being accurately centred, these images 

 are all concentric, and the various pictures can be easily dis- 

 criminated even when two or more of them come into focus 

 together. This method of examining an objective will be found to 

 be a very searching test of its mechanical perfection, for any lens 

 not perfectly set will produce an excentric image. Moreover, this 

 mechanical accuracy in the placing of the lenses is itself a con- 

 dition of high optical quality. A single lens tilted to one side 

 may produce but little effect in the ordinary working of an 

 objective. But it will effectually prevent the instrument from 

 yielding the finest results of which its combination is capable. 

 This test, therefore, is of considerable value, and it has the merit 

 not only of being a crucial test, but, in addition, of being one which 

 indicates the nature of any defect detected. It will therefore, I 

 imagine, be found to be a useful addition to the arsenal of the 

 instrument maker, as well as an easily available test by which 

 the microscopist can examine the mechanical perfection of his 

 objectives. 



The lenses of the objective having been in this way examined, 

 we may now restore the substage, condenser and iris -diaphragm to 

 their places. Then, of course, we shall have to work "with focused 

 light, and the appearance presented by the various images in the 

 mercury globule will be altered accordingly. It will be found that 

 there are two positions of the substage condenser in which definite 

 images beside the image of the mercury globule are given. In one 

 of them, the image is an image of the source of light ; in the other, 

 it is an image of an aperture of the condenser, defined as a rule, 

 of course, by the iris diaphragm. The mercury globule and the 



