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PHOTO-MICROGRAPHY 



recently in the hands of Mr. Conrady. We have only been able to test a few of the 

 lenses, because his series is not yet complete, but those that have been tested show, 

 besides an excellent performance on all the recognised test objects, a very great 

 excellence in one particular, and it is this : The ordinary achromatic will not stand 

 eye-piecing beyond five or seven diameters at the most without producing what is 

 technically known as a "rotten" image, but the objectives in question will work 

 easily with ten, and, using suitable objects, even with an eye-piece having the initial 

 power of magnifying eighteen diameters. Here we have, then, a decided improve- 

 ment, and we understand that it is dependent upon an entirely different line of 

 thought in their construction. Mr. Conrady considers, cceteris paribus, that the real 

 improvement in the construction of achromatics of the future will lie in the careful 

 uniting of equal phases of the rays to be joined in one focus, and inasmuch as his 

 results so overpower those hitherto obtained, his theory, although not easily grasped, 

 may be worthy of the most careful consideration. Notwithstanding what has been 

 said, some very fine lenses do already exist, such as No. 2, by Leitz, Nos. 6 and 7 A, 

 by Peichert, the iVth immersion also by Leitz, and what used to be called the semi- 

 apochromatics by Reichert, let alone some excellent objectives recently brought out 

 by the old-established firms of Ross, Beck, Wray, and others.* 



But before making a selection we must first pause to consider how we are going 

 to photograph at all with a visually constructed objective. Let us suppose, for 

 instance, we put a visually constructed inch on to the microscope, focus, and take a 

 photograph. We should find that although focussed on the ground glass so carefully 

 every structure was absolutely out of focus on the plate. The reason of this is not 

 far to seek. Microscopical objectives computed for visual use are all constructed to 

 give their image by the union of the yellow or yellow-green rays of the spectrum, 

 because the eye perceives this colour more powerfully than any other, hence it is 

 obvious, inasmuch as each colour in the achromatic brings its image to a focus in a 

 different plane, that if we focus in yellow the image produced by the violet rays 



* Whilst these pages are passing through the press, we have had two more new lenses to critically examine, one, an 

 |th immersion by Swift and Son ; and another, a yV^h immersion sold by Baker, of Holborn. 



The |th by Swift is an admirable lens and very well corrected for colour. As an achromatic we have no hesitation in 

 saying that its performance leaves little to be desired. Its N.A. is i'25, and it is very cheap. It is a fair photographer. 



The xVth immersion sold by Baker is perhaps the finest jVth achromatic lens we have ever seen. It rivals a true 

 apochromatic, and with low powers none but an expert could distinguish it from one built upon that principle. Its 

 correction for colour is amazingly good, whilst its defining power leaves nothing to be desired so long as high eye-pieces 

 are not employed, We think the lens has a great future before it, and we most heartily congratulate the firm upon being 

 able to sell it at such a moderate price. Its N.A. is over 1-30, probably about i"33, which for an achromatic is perhaps 

 the highest obtainable. Photographs taken with it — provided a monochromatic screen be employed — compare very 

 favourably with those produced by apochromatics, 



