1. A method of grading tubules in a first image of a histological slide, the method having the steps of: a) providing a second image of first objects in the first image which are sufficiently large and of appropriate pixel value characteristics at boundaries to potentially be tubules, b) providing a third image of second objects in the first image having pixel value characteristics of fat and holes within tubules, c) combining data from the second and third images to identify selected second objects which are within first objects, d) performing one or more of the following: i) counting first objects in the first image which may potentially be tubules to provide a parameter NOB, ii) counting the first objects having selected second objects within them and likely to be tubules to provide a parameter N, iii) determining the relative areas of selected second objects as proportions of respective first objects within which they are located to provide parameters RATIO, iv) determining the total area of selected second objects as a proportion of total area of first objects within which they are located to provide a parameter SURF, v) determining a parameter PERCENT=N/NOB, and vi) counting the number of first objects containing at least medium sized holes to provide a parameter T, and e) grading the first image's tubules on the basis of the one or more parameters as aforesaid with reference to parameter threshold values.