The following ultraviolet absorption maxima have been measured:

1, 3-Butadiene 217 nm

2-Mcthyl-1, 3-butadicne 220 nm

1, 3-Pentadiene 223 nm

2, 3-Dimethyl- 1, 3-hutadiene 226 nm

2, 4-Hexadiene 227 nm

2, 4-Dimethyl-1, 3-pentadiene 232 nm

2, 5-Dimethyl-2, 4-hexadiene 240 nm

What conclusion can you draw about the effect of alkyl substitution on UV absorption maxima? Approximately what effect does each added alkyl group have?