THE USE OF SHADOW-CASTING TECHNIQUE FOR MEASUREMENT OF THE WIDTH OF ELONGATED PARTICLES

A method is described for the estimation of the true width of fibrillar or rod-like structures from electron micrographs of metal-shadowed preparations. The method is based on variations in the image width as a function of the angle (ß) between the long axis of the fibril and the direction of the shadow in the plane of the preparation. The image width when ß = 0° practically represents the real width of the elongated particle but is often indistinguishable from the background. The fibril image width is conveniently measured at ß values between 15° and 90°. The true width is obtained by plotting the image width versus sin ß and extrapolating to ß = 0°. Latex spheres are sprayed with the fibrils or rods to indicate the direction of shadow. Tobacco mosaic virus (TMV) was used as a model structure because of its known constant diameter of 150 A (5). The width (in the case of TMV equal to the diameter) found by the present method was 150 A ± 8 A.