3io 



NATURE 



[September 7, 191 1 



to proceed to obtain a uniform background, and how 

 to adapt and regulate his screen or filter so as to 

 increase or diminish contrast; no explaining to him 

 such things as how to distinguish between the use of 

 an ordinary inch on his microscope, and the employ- 

 ment of a planar or such-like combination of the same 

 magnifying power; no helping him to make progress 

 from low-power work to high-power magnifications, 

 and the pointing out to him the many pitfalls he may 

 expect in this section of the work. Nothing, or very 

 little indeed of this nature is given ; anyhow, in a 

 collective and succinct form. 



Proceeding seriatim through the book, the introduc- 

 tion over, the reader is told first about the microscope 

 and general equipment, not so much in particular for 

 any special type of work — such as high, low, or 

 medium magnifications, but in a general way; and 

 tables of magnification, cuts of different eye-pieces 

 and such-like, occupy several pages. It must not be 

 omitted to mention some good photographs of Abbe's 

 test-plate, but we question the expediency of compar- 

 ing the photographic performance of achromats with 

 that of apochromats, unless the former are used in 

 conjunction with a suitable screen to cut off the 

 secondary spectrum, which we are unable to find was 

 done in this instance. A student upon seeing these 

 comparisons might be led to think that the achromat 

 was of no service at all, whereas the image in the 

 preferred colour with most of the newer form of 

 modern achromats is of the finest description, and 

 almost rivals that produced by the apochromat : but a 

 screen must be used. 



Whilst referring to the resolution possessed by com- 

 binations of different numerical aperture, an error has 

 crept in on p. 60. The 80,000 should be 40,000, as a 

 solid direct cone is being spoken about, and not oblique 

 light. Abbe's law of doubling the number of waves 

 to the inch of the light employed refers only to the 

 use of oblique pencils, and not to direct illumination 

 at all. Passing over this slip in the reading of the 

 proofs, we come to the consideration of collecting 

 systems, and here again we find a most comprehensive 

 chapter full of interest and information ; but we must 

 remind the reader that the Kohler lenses are not of 

 the simple kind that the author leads one to believe. 

 A careful examination of their curves reveals at once 

 that they are of what may be called the distorted type. 

 Their originality and usefulness has led the makers 

 to call them after their inventor's name. 



The sources of illumination, as we have already said, 

 are fully discussed in the chapter devoted to this 

 branch of the subject, and the dark-room and photo- 

 graphic methods, with all connected with the produc- 

 tion of the negative and positive, whether on glass or 

 paper, carefully gone into. The master mind is here 

 very much in evidence, although he offers but scanty 

 information how to distinguish a slightly over-exposed 

 from a slightly under-exposed plate; his opinions 

 would have been welcome we feel sure, even to the 

 experienced hand, for at times it is extremely difficult 

 to decide. 



Several methods of reducing vibration with appara- 

 tus located in other places than the basement are 

 mentioned, but we did not notice the method of using 

 NO. 2184, VOL. 87] 



several layers of carpet-felt under each leg of the 

 apparatus. We have been told — and it is common 

 knowledge — that several workers in busy streets 

 employ this method — which was suggested by Mr. 

 Norman — with much satisfaction. 



A chapter is written upon the manipulation of the 

 apparatus in general, but it is of a scattered nature 

 and not collected under specific heads, so that the 

 reader is left to ferret out what he wants for any 

 special kind of work. The information may be there, 

 but not easy to find. 



The use of filters and screens is dealt with, and, so 

 far as it goes, is of great value and information ; but 

 it is to be regretted more particular mention is nut 

 made of the uses of the box of " gelatines " sold by 

 Messrs. VWatten and Wainwright, of Croydon. In 

 the booklet accompanying the arrangement opposite 

 to the name of most dyes used for microscopy, is 

 placed the number of the screen to be used to increase 

 contrast if required, whilst, in addition, besides show- 

 ing how to reduce contrast, a table is given showing 

 how by the assembling of certain of the same films 

 together, monochromatic light of given wave-length 

 can be obtained at almost any part of the visible 

 spectrum. It is obvious of what service this little 

 " box " is to the practical man at work, and ought to 

 be always named as part of the equipment. 



The description of several special processes, stereo- 

 scopic photography, the photographing of culture tubes 

 and cultures, are followed by some important remarks 

 concerning the use of ultra-violet rays ; and then 

 comes a very comprehensive index and the plates. 



We are bound to express our disappointment at the 

 appearance presented by plate i. Everyone knows 

 it is not altogether easy to obtain a uniformly and 

 pleasantly tinted background, but in this instance the 

 blocks have been cut away up to the very margin of 

 the object, and even that is sliced up into moieties ! 

 Surely a student who looks for guidance wants an 

 ideal to live up to, a standard at which to aim? 1 1 

 the teacher be guihv of exhibiting work of this descrip- 

 tion, how will he teach the tyro to do better? The 

 author's strong point is certainly not that of photo- 

 graphing diatoms, but is rather that dealing with 

 bacteriological specimens and such like. Take, for 

 example, the photograph of Trypanosoma gambiense, 

 or that of Bacillus typhosus, in plate vii. : we doubt 

 if it be possible to find two more magnificent illus- 

 trations in any book upon the subject. So too with 

 the Podura scales in plate viii. The manner in which 

 the constriction around the head of the so-called 

 "comma" or "note" is shown is beyond all praise, 

 and so too is the delicate tapering off to a point of 

 its extreme end : it is perfection. We wonder what 

 the promoters of the new idea that this focus is not 

 the correct one, and that they have found another 

 two or three (according to how they misarrange 

 their adjustments), will say to this magnificent work? 

 Taking " Practical Photomicrography " as a whole, 

 we cordially recommend it, and congratulate the 

 writer; but the reader must approach it as a dictionary 

 full <>l answers lo questions he may desire to put, 

 rather than as a guide, philosopher, and friend, for that 

 it is not. 



