14 METHODS. 



within a reasonable space of time, to take the many thousand measurements 

 necessary for this by means of the methods hitherto employed. 



I therefore cast about for a better method and finally worked out a new 

 micromeasuring apparatus, which Mr. Agassiz's Uberality enabled me to set 

 up. The plan of this apparatus (Fig. 1) has proved most useful. The greater 

 part of it is also represented on Plate 109. The light of a powerful, self-regulat- 

 ing constant-current arc-light (Fig. la) passes a system of lenses and cooler 

 (Fig. lb) and enters a microscope (Fig. Ic) with the optical axis placed horizon- 

 tally. A movable mirror 1.5 X 1.5 m. in size (Fig. Id) is so placed in front 

 of the microscope that the image produced is reflected on to a vertical glass- 

 plate (Fig. le) frosted on the side turned towards the mirror, and measuring 

 2 X 2 m. Lamp and microscope are so placed that the latter stands at the 

 side of and close to the frosted glass-plate. The observer sitting in front of 

 the latter can comfortably work both the screws moving the sHde to right 

 and left and up and down (Fig. Ig), and that focussing the microscope (Fig. Ih). 

 The horizontal optical axis of the microscope is oblique (not vertical) both to 

 the mirror and the frosted glass. The mirror, however, is placed so that the 

 axis of the cone of Ught reflected from it abuts vertically on the frosted glass- 

 plate. This arrangement insures the image, thrown on to and visible on the 

 frosted glass, being perfectly true, and not in any way distorted. By means 

 of the screws moving the slide, everything on it can easily be passed in review. 

 When a spicule, or anything else that is to be measured, comes into view, it 

 is focussed and measured. 



When working with this apparatus, I placed lamp, microscope, mirror, and 

 frosted glass always in the same position; and every time I commenced I tested 

 the correctness of the position of the parts by projecting and measuring the 

 micrometer-slide. For each combination of objectives and eyepieces used I 

 made a special scale which was drawn on a ribbon of tracing-cloth. These 

 ribbons (tapes) were fixed to canes, like strings to bows (Fig. 2). It is easy 

 with these bow-string tapes to measure rapidly the distance between any two 

 points in a plane vertical to the optical axis of the microscope. 



The observer dictates the dimensions thus measured, and anything else 

 notable he may observe. His assistant sits behind him at a table (Fig. li) 

 with a shaded (Fig. Ik) light (Fig. 11). It is possible, if the preparation is a 

 good one, to write down the dimensions at the rate of six to ten per minute. 



This method is not only convenient and rapid, but also exceedingly accurate. 

 The measurements taken with it when using high powers are exact to 0.1 m- 



