ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 137 



which have brought the instrument to its present state of perfection. It 

 would, indeed, take several evenings to systematically examine the 

 great number of forms which have been introduced since that time. It 

 is my intentien, however, only to notice three, as most of the others, not 

 being of any practical value, have speedily become obsolete. We need 

 no diagrams of the three forms which have survived, as I have actual 

 examples in the room. First there is this, which is known as the 

 " Hartnack," or " Continental Model," it is a lineal descendent of the 

 " Oberhauser." I have little hesitation in saying that nine-tenths of all 

 original microscopical work has been done by these Microscopes, but at 

 the same time I maintain that that statement does not prove it to be the 

 best model. It is a model which is incapable of doing critical work 

 with low powers, and of working any high power at all. The reason 

 why so many discoveries have been made with it is due to the fact that 

 nine-tenths of the things discovered lie among low-power objects. 

 Another point must be borne in mind, viz. that a quarter-inch 

 lens uncritically used will as readily discover an object as a half-inch 

 critically used. 



The interpretation of images with low powers is easy, and requires 

 very little training; critical images, therefore, are not so essential. 

 Most of the fine high-power work which has been carried on with these 

 instruments has been erroneous, and has had to be corrected with other 

 instruments. As time goes on, discoveries with the low powers become 

 less and less possible, and instruments of greater precision will become 

 necessary." 



" The importance of a condenser cannot be over-estimated. I have 

 always held that Microscopy begins with a condenser. An instrument 

 however well designed and well constructed, if it has not a condenser, 

 is nothing more than a magnifying glass, while on the other hand, a 

 simple stand like this iron one of Powell's, with a condenser, forms 

 a very efficient Microscope." 



' Student's Handbook to the Microscope.' * — This little book ful- 

 fils its purpose in a very creditable manner, and will be a useful guide 

 for a large number of Microscope owners. It is a decided advance on 

 the author's previous venture, 'My Microscope,' the publication of 

 which was, we thought, to be regretted. 



Even in these days it is, we suppose, hopeless to expect the question 

 of aperture to be dealt with without a mistake, and therefore we find on 

 p. 37, the statement that among the drawbacks to an excess of aperture 

 is " a loss of defining power, that is distinctness of the image." This 

 arises from an entire misunderstanding of the principles of aperture. 

 The larger the aperture, the less the penetrating power, or the power of 

 seeing a given depth of the object with the same focus. But the definition 

 of the particular plane, whatever its depth, which is seen by the large 

 aperture is not in any way impaired ; in fact the definition of what is 

 seen is more complete and perfect with the " high angle " objective than 

 with one of smaller aperture. 



"Microscopical Advances." f — " T. F. S.," writing on one of a 

 series of articles under this heading by Dr. G. W. Eoyston-Pigott, 



* A Quekett Club-man, ' The Student's Handbook to the Microscope. A 

 Practical Guide to its Selection and Management,' vii. and 72 pp. (30 figs.) 8vo, 

 London, 1887. t Engl. Mech., xlvi. (1888) p. 435. 



