THE PURCHASE OF A MICROSCOPE 261 



of an optical combination might in that instrument take care of itself, 

 but a diaphragm aperture must be centred by mechanism and 

 focussed by rack. 



We know that the idea involved in a rack- work diaphragm is 

 the graduation in the angle of the cone- of illumination from the 

 plane mirror by racking a certain-sized diaphragm up or down. 

 But this can be better done by an iris diaphragm, or perhaps more 

 perfectly still by a wheel of diaphragms. 



Now, in reality nothing is so important as the centring and 

 focussing of the condenser, after we are once provided with perfect 

 objectives ; and any mechanical arrangement that would enable us 

 to perfectly centre an iris diaphragm or a wheel of diaphragms 

 would enable us to centre the condenser. For the racking and 

 centring of condensers there was, until very recent times, nothing 

 in the best stands, of what is, doubtless the largest and most 

 enlightened house for the manufacture of microscopes in the 

 world, to supply this indispensable need which the modern con- 

 denser involves. 



We observe with pleasure advances in every direction in which 

 we have called attention to defects. The more recent instruments 

 are marvels of ingenuity; we present, in fig. 167, the latest and 

 finest form of Zeiss's best microscope. 



There is no fault in the workmanship ; it is the best possible. 

 The design only is faulty, there is nothing to command commenda- 

 tion in any part of the model ; and, seeing that the Messrs. Zeiss 

 have now progressed so far as to furnish their first-class stand with 

 the English mechanical movement, and even stage rotation, and fine 

 adjustment to their newest and best sub-stage condenser, we can 

 but believe that the advantages of these improvements will make 

 plain the greater advantage that would accrue from an entirely new 

 model. To all who study carefully the history of the microscope 

 arid have used for many years every principal form, it will, we 

 believe, be manifest that the present best stand of the best makers 

 of the Continent is an over-burdened instrument. Its multiplex 

 modern appliances were never meant to be carried by it. The 

 attempt to combine a dissecting microscope with an observing 

 microscope required to do the most critical work is not, we submit 

 with all friendliness, compatible. 



The Purchase of a Microscope. A desire to possess a good but 

 not costly microscope is extremely common, but as a rule the 

 intending purchaser has little knowledge of the instrument, and 

 does not profess to know what are the indispensable parts of such an 

 apparatus, or what parts may, in the interests of economy and his 

 special object, be dispensed with, leaving him still possessed of 

 a sound and well-made instrument. We may briefly consider this 

 matter. 



The first question to be asked when a microscope is to be pur- 

 chased is, * What is the order of importance of the various parts of 

 a microscope ? ' In answering this query it will be to some extent 

 true that subjectivity of judgment will appear. But we believe 

 that the following table of the relative order of importance of the 



