MAGNITUDE OF MICROSCOPIC OBJECTS. 211 



glass of proper size placed over fig. 3 : and if the lines across 

 and near the centre are drawn by very light touches, so as to 

 be scarcely visible to the unassisted eye, the centreing of the 

 instrument is more easily effected, while no perceptible defect 

 results in the illumination of the object. 



More elaborate instruments, possessing movements in hori- 

 zontal and vertical directions by means of fine screws with 

 micrometer heads, have already the powers requisite for 

 defining the place of an object, when once the commencing 

 position, fig. 1, has been carefully ascertained, and either 

 marked upon the instrument or registered for reference. 



The principle, therefore, is simple in its cliaracter as well 

 as perfectly general in its application, and supplies the want, 

 which has been expressed, of a " Universal Indicator " for 

 the microscope. 



With reference to the measurement of the magnitude of 

 microscopic objects, I have to suggest a modification of the 

 ingenious and elegant micrometer of Welcker, described in 

 No. XIV. of the Microscopical Journal, by which all gradua- 

 tion is dispensed with, except such as is found upon the 

 ordinary scales supplied with the commonest sets of mathe- 

 matical instruments, viz., a scale of half-inches divided to 

 tenths and hundredths. By means of cross- lines drawn on 

 the diaphragm of the eye-piece, and with a stage micrometer 

 divided to hundredths and tliousandths of an inch, the radius 

 of the circle traced out by the intersection of the cross-lines 

 is carefully measured. The positions CD, cff (fig. 4), show 

 the method of effecting this ; and if it should be found that 

 the radius of the dotted circle does not coincide exactly with 

 some number of thousandths of an inch, this inconvenience 

 may be easily rectified by means of the draw-tube. For 

 example, in an instrument which I have recently applied to a 

 Student's Microscope by Messrs. Smith and Beck, the radius 

 of the dotted circle was found to be • 0145 inch very nearly : 

 by drawing out the tube to the extent of 3" 4 inches the radius 

 was measured exactly by • 01 inch. 



The modification which I propose for the external part of 

 Welcker's instrument consists in substituting a right-angled 

 triangle for the circular sector, and suppressing all graduation. 

 A glance at fig. 5 will explain the matter at once. The angle 

 at E is a right angle, and the distance O E is exactly five 

 inches. The method of measuring the object is shown in 

 fig. 6. The object is brought so that one extremity of it is at 

 the intersection of the cross-lines, while the index O F coin- 

 cides with the line O E on the triangle. By the rotation of 

 the eye-piece, the diametral line is made to touch the other 



