CORRESPONDENCE. 43 



their criticism of the way in which the English Jnrors passed over their 

 work at the Paris International Exhibition. I well remember being 

 there myself, but because the Test-object I was exhibiting was un- 

 known to the said Jurors, I was scarcely noticed by them. I heard 

 too, that an English oj)tician criticised us with some acrimony in the 

 ' Athenaeum '; he went so far as to say that the Test-object, which I was 

 exhibiting with a magnification of about twelve hundred diameters, was 

 altogether too coarse to use as a test even for a low power ; — but he 

 was beside the mark. 



Very soon after this, myself and other Immersions were pitted 

 against some of the then best Dry Objectives in England, and we more 

 than held our own. The merits of our principle of construction being 

 thus brought prominently to the notice of English amateurs and opti- 

 cians, some of the latter were not slow in taking the matter iip. Then 

 came the announcement that Dr. Woodward had made a series of 

 photographs of Nobert's new Nineteen-band Test-Plate with Messrs. 

 Powell and Lealand's y^*^ Immersion. This was immediately 

 followed by copies of the photographs which were exhibited at the 

 Royal Microscopical Society. On comjiarison with the photographs 

 of the same Test-plate made with some of the finest and most power- 

 ful Dry Objectives it was abundantly evident that the Immersion 

 princif»le would now take the lead. Dr. Woodward, with a zeal and 

 perseverance that were truly admirable, followed up these photographs 

 by others of the Podura, AmjjJiipleura pellucida, FrustuUa saxonica, 

 Bhomboides, &c., &c., produced by various Immersion lenses ; and 

 these photographs can be referred to in the collection of the Royal 

 Microscopical Society by any one interested in the subject. 



So far as I am concerned personally I shall be glad to have our 

 merits judged by our works. On the part of many of my fellow Im- 

 mersion lenses and myself I accept Dr. Woodward's photographs as 

 fairly representing our capabilities up to the date of their produc- 

 tion ; and I venture to believe that in them we show results ahead of 

 anything that has hitherto been done of a similar kind by any Dry 

 Objective. 



Since that date, and notably as exhibited at the Vienna Exhibition, 

 improvements have been made in our construction ; our younger 

 brethren have had a fourth combination added ; that is, have a single 

 front (which the oldest of us have had — aye, even that made by 

 Dr. Brewster in the beginning of the century ; — I hope Mr. Wenham 

 will pardon this slur on his claim to be the inventor of single fronts !) 

 and three doublet achromatics progressively increasing in diameter. 

 In this construction a zone of peripheral rays is gained and made 

 available in the formation of the image ; — which we believe will be 

 an advantage in high powers. 



The close study of M. Pouillet's experiments to determine the 

 conditions of the production of diffraction in the passage of light 

 through various forms and parts of prisms has led one of the most 

 learned of the Paris opticians to aim at extending as much as prac- 

 ticable the introduction of marginal rays into the formation of the 

 image : the new objectives of four combinations that were shown at the 

 Vienna Exhibition were practical examples in this direction. 



