MAGNIFYING TOWER. 



Let a micrometric scale, such as we have already described, be 

 placed on the stage, the instrument focussed, the camera attached, 

 and a sheet of paper placed at 10 inches from it. An image of 

 the scale being seen on the paper, let any two contiguous divisions 

 of it be marked with the pencil. Let the distance between these 

 marks be then exactly measured, and let it be divided by the 

 .actual length of the divisions of the scale. The quotient will be 

 the magnifying power. 



Thus, for example, let us suppose that the micrometric scale is 

 the 25th part of an inch, and that this length is divided into 100 

 parts, each of these parts will be the 2500th part of an inch. Now 

 suppose that it is found that the distance between the images of 

 two contiguous divisions on the paper, is four-tenths of an inch. 

 It will follow that the visual magnitude of a division of the scale 

 is magnified in the proportion of ^ to , that is, as 1 to 1000. 

 The magnifying power would therefore be a thousand. 



There are other methods of ascertaining the magnifying power, 

 but this is so simple, so easily produced, and so precise, that we 

 shall not detain the reader by any notice of others. 



Microscopes being generally supplied with several object- 

 glasses, and eye-pieces, the observer and amateur would do 

 well once for all to ascertain the magnifying powers of all the 

 possible combinations of them, and to tabulate it and keep it for 

 reference. 



68. It is often asked, What are the dimensions of the most 

 minute object which a microscope, having a given magnifying 

 power, is capable of rendering distinctly visible ? 



The answer to this question will depend on the answer to 

 another ; What are the least dimensions of the same object, with 

 which it would be distinctly visible, at ten inches distance, with 

 the naked eye ? 



Whatever be the latter dimensions, the former will be just so 

 many times less as there are units in the number which expresses 

 the magnifying power. 



Thus, for example, if the smallest linear dimensions with 

 which the object could be distinctly seen without a glass at 10 

 inches distance were the 300th part of an inch, a microscope 

 having a magnifying power of 500 would render such an object 

 equally visible if its linear dimensions were only the 300 X 500 

 = 150000th part of an inch. 



It is generally considered that the smallest disc of which tha 

 form can be distinguished by the naked eye, being properly con- 

 trasted with the ground upon which it is seen, is one which would 

 have a visual angle of one minute ; and since a line measuring 

 the 360th part of an inch, placed at ten inches distance, would 



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