78 A STANDARD MICROSCOPE 



The base measures 6^ x 4 x 1 inch. 



The distance of the stage from the table is 4| inches, which 

 allows more room for substage apparatus than has been generally 

 given. 



The diameter of the mirrors is 2 inches, and they have a vertical 

 adjustment of H inches. 



The stage is 4 inches across, and there is a free distance of 

 3 inches between the optic axis and the limb. 



The instrument will carry the ordinary double and triple nose- 

 pieces, but we have taken up a new object glass changer invented 

 by Mr. Sloan, of Birkenhead, which we have found by prolonged 

 use to possess many advantages over a revolving nose-piece, and by 

 putting down tools we have been able to produce it at a very 

 moderate price. The design is so simple and rigid that almost 

 absolute accuracy of centering can be permanently maintained, and 

 the errors of mounting of individual object glasses can be compen- 

 sated. TTiere are no slides, but the adjustment throughout is made 

 by screwed abutment pins with clamping screws. Once these are 

 adjusted and fixed they cannot shift, and the utmost error we have 

 been able to detect in the alignment of the optic axis by the tightness 

 or looseness with which the clamp by which the object glass and its 

 fitting is secured to the microscope is about 1/6 part of the field 

 of 1/6. 



We have introduced a new micrometer eye-piece and a new system 

 of measurement which appears to be in advance of previous methods. 

 The object glasses are all engraved with an initial magnifying power, 

 which is the magnifying power at the first image formed by the 

 object glass with a tube length of 160 mm. We have designed a 

 new vernier scale for measuring objects, with a special positive 

 €iye-piece which is entirely above the scale, and when this is placed 

 in the microscope the scale is in the exact position occupied by the 

 image which is formed by the object glass when the medium power 

 eye-piece is used. The object under examination is measured in 

 1/10 of a millimetre on this scale, and the result divided by the 

 figure engraved on the object glass gives the actual size of the object. 

 If a stage micrometer be placed under the microscope, the initial 

 magnifying power of the object glass may be checked, though this 

 will only be necessary for very exact work. If a Sloan object 

 changer is used, the drawtube must be set to 150 mm., or if a 

 nose-piece is used it must be set to 145 mm. to compensate for the 

 increase in tube length produced by these pieces of apparatus. 



