Let $X$ be $\mathbb{P}^{2}$(over $\mathbb{C}$) blowing up at 7 points in general position. Denote the points as $p_{1}, p_{2}, …,p_{7}.$ Denote the blowing up map as $\pi_{1}: X\to \mathbb{P}^{2}$.
We know there are 56 exceptional curves on $X$.
7 of them come from the total transform of $p_{1}, p_{2},…,p_{7}$, denote them as $E_{i}$
21 come from the strict transform of the lines passing through $p_{i}p_{j}$, denote them as $E_{ij}$
21 come from the strict transform of conics passing through 5 of the 7 points, denote them as $D_{ij}$, where the conic does not pass through $p_{i}, p_{j}$
7 come from cubics passing through all of 7 points and has double points at $p_{i}$, denote them as $D_{i}$.
It is well known that $X$ also could be realized double cover of $\mathbb{P}^{2}$ ramified at smooth quartic. Denote the double cover as $\pi_{2}: X\to \mathbb{P}^{2}$. The $X$ has an order 2 group action, denote the nontrivial action as $\sigma$. Then $\pi_{2}: X/<\sigma>\to \mathbb{P}^{2}$.
The 56 exceptional curves also come from 28 bitangents of the quartic.
It is not hard to show, actually $\sigma(E_{i})=D_{i}, \sigma(D_{i})=\sigma(E_{i})$. $E_{i}, D_{i}$ comes from the same bitangent line of the quartic.
Now my question is if we pick the general line $l$ on $\mathbb{P}^{2}$ which does not pass through any of 7 points, denote the preimage under the blowing up as $l_{1}$, what $\sigma(l_{1})$ looks like? 
Actually $l_{1}E_{1}=0, l_{1}D_{1}=3$, thus $\sigma(l_{1})E_{1}=3, \sigma(l_{1})D_{1}=0$. However, I could not find a rational curve which intersect $E_{1}$ at 3 points and does not intersect with $D_{1}$.
