Negative slide showing a line chart of 'exhibition of NH group: tenth day classification'. Negative slide shows a line chart of 'exhibition of NH group: tenth day classification'. X axis shows 'time in days' from 1 to 13; the Y axis is labelled 'index of exhibition', and the scale is from 0 to 0.9. There are five lines in the chart which all start between 0.8 and 0.9 on the Y axis. The lines are identified by different symbols which are described in the key below. Title below image reads: 'Exhibition of NH group: tenth day classification. [white circle] class I; [blank circle] class II; [white square] class III; [blank square] class IV; + class V.' Late 19th to early 20th century