186 THE pr.EST dent's address. 



metal in the manufacture of the substage and the apparatus 

 which it supports. 



Turning now to the optical portion of the microscope, there is 

 no doubt that within the last few years most important advances 

 have been made towards perfection. Even during the last twelve 

 months, or at any rate within the last two years, more than one 

 optician has added a contribution to the resources of the micro- 

 scopist. By the use of several different varieties of Jena glass 

 opticians are daily vieing with one another in the perfecting 

 of apochromatic objectives without the use of fluorite in their 

 construction. Only recently a new glass of y\j-inch focal length 

 and of 1-30 N.A. has been constructed by Leitz, of Wetzlar. 

 Several of these glasses, constructed for the long as well as for 

 the short tube, have found their way into this country, and 

 from the specimens I have seen I am bound to speak highly 

 of their performance. When used with a malachite green screen, 

 the best of these glasses give an image of remarkable brightness, 

 capable of standing fairly high eyepiecing, without appreciable 

 loss of definition. This glass is sold at a very moderate price : 

 in fact, it is, as far as I know, the cheapest glass of the kind 

 hitherto produced. Another objective, which, in its present state 

 of perfection, is new at any rate to me, deserves special mention 

 here. It is a y\rth of hard glass, specially constructed for use 

 in hot climates, by Keichert, of Vienna. It has an aperture 

 rather greater than that of the y'^ th first spoken of — perhaps of 

 about 1*35. This objective also is one of exceptional merit ; it 

 is a very strong resolver, and produces an image of great purity 

 and brilliancy, which bears eyepiece amplification remarkably 

 well. It has, however, in my judgment, the important defect 

 that it is adjusted only for a very short tube. If this objective 

 could be corrected for the ten-inch tube, and made to work as 

 wtU on this as it does on the short tube, I am sure that it would 

 be found to compare favourably with the majority of fluorite 

 apochromatics, although the latter cannot be purchased for less 

 than double the price which is at present charged for this 

 objective. 



I believe that I am correct in stating that the achromatic 

 condenser is regarded, by competent authorities, to be the one 

 accessory which is absolutely essential, if the compound microscope 

 is to occupy any better position than that of a costly toy. And 



