436 Transactions of the Society. 



One of these measures - 53 N.A. This was the last effort to im- 

 prove the non-achromatic dioptric Microscope. 



Having described at length the three forms of Microscopes, the 

 question remains : What did their users see through them ? We 

 have examples of simple and compound dioptric Microscopes at 

 hand, and therefore with regard to these we may see for ourselves ; 

 but Dr. Smith's catadioptric is such a rare form that I have never 

 even seen one, nor known anybody that has, so we must form an 

 estimate of its capabilities from the reports of those who have used 

 it, and from a careful examination of the formula upon which it 

 was constructed. 



Taking the dioptric Microscope first, it has been said that the 

 compound form is inferior to the simple. It is possible that this 

 saying has arisen owing to the numerous discoveries made by the 

 famous Leeuwenhoek (1673) with his simple Microscopes. It is 

 true that for dissecting work single lenses, of powers from that of 

 a watchmaker's eye-glass to a ^-in. focus, are more suitable than 

 any compound Microscope ; nevertheless, for an observing instru- 

 ment there can be no question that the compound Microscope not 

 only gives far better images, but also is much more easy to work 

 with, because the working distance is greater and the field larger. 

 A careful comparison was made between the Benjamin Martin 

 compound with a No. 1 lens and Cuff's ^. Both resolved 15,000 

 ruled lines to the inch, but the resolution with the compound was 

 much stronger and sharper than with the single. A coarse Navi- 

 cula lyra (17,000 per inch) was shown indifferently resolved by 

 both, but the compound gave the better image. Neither would 

 dot that beautiful and easy diatom the Actinocyclus Ralfsii * ; with 



* Probably the two most beautiful of all microscopical objects are the 

 Actinocyclus Ralfsii (the blueish rather than the greenish variety), and the eight 

 rayed Aulacodiscus Kittonii, but these, like everything else, require to be properly 

 shown. The Actinocyclus Ralfsii should be viewed in daylight with a low power 

 of about 0' 3 N.A., and a low power achromatic condenser. If it is too much 

 resolved the beautiful colour fades, if it is not sufficiently resolved the fine ray 

 effect is not seen to advantage ; there is a precise point between these two where 

 the image is just right. The Aulacodiscus Kittonii requires different treatment, 

 only a very fine lens of about 0'65 N.A. will show this extraordinarily beautiful 

 diatom properly. The same low-power condenser is used (an achromatic £5, 

 used with its top off, focus - 8), with daylight, and the plane mirror as before. 

 The substage is racked up until the window-bars are in focus, and a stop about 

 half an inch in diameter is placed in the holder. This stop should be of such a 

 size that it will just give a dark ground with an object-glass of - 3 N.A., and 

 therefore it will not give a dark ground with an object-glass of 0*65 N.A. The 

 important part is the lens adjustment, which will be found to be extremely 

 sensitive with this kind of illumination, one-tenth of an inch alteration of tube- 

 length will, with a critical eye, make all the difference between a good or a bad 

 image. There is a peculiarity in regard to this method of work which must be 

 explained at some length. There are two images, which, for convenience, may be 

 called the positive and negative images ; in the positive image the areolations are 

 darker than the silex, but in the negative the silex is darker than the areolations. 

 No microscopist, however expert, if shown either of these images, would suspect 

 anything wrong with them. They, although exquisitely sharp, are both false 

 images I The method of detecting this is by measuring the tube-length : that 



